I have just updated with Fedup from Fedora 20 to Fedora 21.  

Owing to a driver problem with the RPMFusion proprietary nvidia driver, I have reverted to Nouveau.  Sadly this problem is **STILL** -- YEARS later -- in effect!

I have the same GT240 card.  I can't work for more than about 1 hour when the PC Freezes and the logs show repeated events of:

Feb 23 05:19:02 fedora kernel: [ 2954.013518] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:19:04 fedora kernel: [ 2956.011780] nouveau E[      VM][0000:01:00.0] vm flush timeout: engine 13
Feb 23 05:19:08 fedora kernel: [ 2958.011166] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH TLB flush idle timeout fail
Feb 23 05:19:08 fedora kernel: [ 2958.011173] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_STATUS  : 0x00b00003 BUSY DISPATCH TPC_PROP TPC_TEX TPC_MP
Feb 23 05:19:08 fedora kernel: [ 2958.011182] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS0: 0x00000000
Feb 23 05:19:08 fedora kernel: [ 2958.011186] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS1: 0x00005068 TPC_TEX
Feb 23 05:19:08 fedora kernel: [ 2958.011190] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS2: 0x00000000
Feb 23 05:19:08 fedora kernel: [ 2960.009270] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:19:10 fedora kernel: [ 2962.007477] nouveau E[      VM][0000:01:00.0] vm flush timeout: engine 13
Feb 23 05:19:14 fedora kernel: [ 2964.005890] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH TLB flush idle timeout fail
Feb 23 05:19:14 fedora kernel: [ 2964.005895] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_STATUS  : 0x00b00003 BUSY DISPATCH TPC_PROP TPC_TEX TPC_MP
Feb 23 05:19:14 fedora kernel: [ 2964.005899] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS0: 0x00000000
Feb 23 05:19:14 fedora kernel: [ 2964.005901] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS1: 0x00005068 TPC_TEX
Feb 23 05:19:14 fedora kernel: [ 2964.005904] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS2: 0x00000000
Feb 23 05:19:14 fedora kernel: [ 2966.003981] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:19:16 fedora kernel: [ 2968.002156] nouveau E[      VM][0000:01:00.0] vm flush timeout: engine 13
Feb 23 05:19:20 fedora kernel: [ 2970.000542] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH TLB flush idle timeout fail
Feb 23 05:19:20 fedora kernel: [ 2970.000546] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_STATUS  : 0x00b00003 BUSY DISPATCH TPC_PROP TPC_TEX TPC_MP
Feb 23 05:19:20 fedora kernel: [ 2970.000550] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS0: 0x00000000
Feb 23 05:19:20 fedora kernel: [ 2970.000553] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS1: 0x00005068 TPC_TEX
Feb 23 05:19:20 fedora kernel: [ 2970.000555] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS2: 0x00000000


until it the below happens, and the PC reboots on its own:

Feb 23 05:19:38 fedora kernel: [ 2989.982609] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:19:40 fedora kernel: [ 2991.980777] nouveau E[      VM][0000:01:00.0] vm flush timeout: engine 13
Feb 23 05:19:42 fedora kernel: [ 2992.269810] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH TLB flush idle timeout fail
Feb 23 05:19:42 fedora kernel: [ 2992.365716] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_STATUS  : 0xffffffff BUSY DISPATCH UNK2 UNK3 UNK4 UNK5 M2MF UNK7 CTXPROG VFETCH CCACHE_PREGEOM STRMOUT_VATTR_POSTGEOM VCLIP RATTR_APLANE TRAST CLIPID ZCULL ENG2D RMASK TPC_RAST TPC_PROP TPC_TEX TPC_GEOM TPC_MP ROP (unknown bits 0xfe000000)
Feb 23 05:19:42 fedora kernel: [ 2994.634783] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS0: 0x00000000
Feb 23 05:19:42 fedora kernel: [ 2994.634785] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS1: 0x00005068 TPC_TEX
Feb 23 05:19:42 fedora kernel: [ 2994.634787] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS2: 0x00000000
Feb 23 05:19:42 fedora kernel: [ 2994.634808] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00200000, ch 6
Feb 23 05:19:42 fedora kernel: [ 2994.634823] sched: RT throttling activated
Feb 23 05:19:45 fedora kernel: [ 2997.287884] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:19:45 fedora kernel: [ 2997.287897] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00200000, ch 6
Feb 23 05:19:48 fedora kernel: [ 2999.950671] Clocksource tsc unstable (delta = -95866636 ns)
Feb 23 05:19:48 fedora kernel: [ 2999.950673] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00200000, ch 6
Feb 23 05:19:48 fedora kernel: [ 2999.950684] nouveau E[      VM][0000:01:00.0] vm flush timeout: engine 13
Feb 23 05:19:48 fedora kernel: [ 2999.951748] Switched to clocksource hpet
Feb 23 05:19:54 fedora kernel: [ 3006.589343] nouveau W[  BARCTL][0000:01:00.0] flush timeout
Feb 23 05:19:54 fedora kernel: [ 3006.589377] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:19:56 fedora kernel: [ 3006.926383] hrtimer: interrupt took 288028945 ns
Feb 23 05:19:56 fedora kernel: [ 3007.917049] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x04800000, ch 6
Feb 23 05:19:57 fedora kernel: [ 3009.243384] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:19:57 fedora kernel: [ 3009.243414] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:19:58 fedora kernel: [ 3010.569768] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:19:58 fedora kernel: [ 3010.569789] nouveau E[      VM][0000:01:00.0] vm flush timeout: engine 13
Feb 23 05:20:01 fedora kernel: [ 3013.229411] nouveau W[  BARCTL][0000:01:00.0] flush timeout
Feb 23 05:20:01 fedora kernel: [ 3013.229478] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:20:02 fedora kernel: [ 3014.548819] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:20:02 fedora kernel: [ 3014.555772] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x04000000, ch 6
Feb 23 05:20:03 fedora kernel: [ 3014.784704] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH TLB flush idle timeout fail
Feb 23 05:20:03 fedora kernel: [ 3014.880615] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_STATUS  : 0xffffffff BUSY DISPATCH UNK2 UNK3 UNK4 UNK5 M2MF UNK7 CTXPROG VFETCH CCACHE_PREGEOM STRMOUT_VATTR_POSTGEOM VCLIP RATTR_APLANE TRAST CLIPID ZCULL ENG2D RMASK TPC_RAST TPC_PROP TPC_TEX TPC_GEOM TPC_MP ROP (unknown bits 0xfe000000)
Feb 23 05:20:03 fedora kernel: [ 3015.875153] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS0: 0x00000000
Feb 23 05:20:03 fedora kernel: [ 3015.875157] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS1: 0x00005068 TPC_TEX
Feb 23 05:20:03 fedora kernel: [ 3015.875161] nouveau E[  PGRAPH][0000:01:00.0] PGRAPH_VSTATUS2: 0x00000000
Feb 23 05:20:03 fedora kernel: [ 3015.875172] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:20:05 fedora kernel: [ 3016.110993] nouveau E[  PGRAPH][0000:01:00.0] vm flush timeout
Feb 23 05:20:05 fedora kernel: [ 3017.201529] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6
Feb 23 05:20:06 fedora kernel: [ 3018.527886] nouveau W[   PFIFO][0000:01:00.0] unknown intr 0x00800000, ch 6