After upgrading to Feisty I have the same Problem. When I left my computer for several hours alone  the Desktop seems to work (Clock changes, Knewsticker is running) but when I move the mouse the Display freeze and the Keyboard stops working. In some cases the effect could happen after a few minutes. 2 days ago, I disabled the Screensaver, but the Bug still occurs.

The xorg process is running with nearly 100% CPU Time. An strace shows, that it loops on this command:

--- SIGALRM (Alarm clock) @ 0 (0) ---
sigreturn()                             = ? (mask now [])

dmesg shows something interesting:

[  153.572298] NET: Registered protocol family 4
[  153.636382] NET: Registered protocol family 3
[  153.727573] NET: Registered protocol family 5
[  159.658624] ISO 9660 Extensions: Microsoft Joliet Level 3
[  159.931681] ISO 9660 Extensions: RRIP_1991A
[36590.032279] NETDEV WATCHDOG: eth1: transmit timed out
[36590.032286] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=2035.
[36590.480003] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf26
[36598.474546] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf27
[36606.469088] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf28
[36614.463632] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf29
[36622.458174] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf2a
[36630.452719] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf2b
[36638.447264] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf2c
[36646.441807] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf2d
[36654.436351] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf2e
[36662.430896] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf2f
[36670.425442] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf30
[36678.419985] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf31
[36686.414530] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf32
[36694.409075] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf33
[36702.403618] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf34
[36710.398162] NVRM: Xid (0005:00): 16, Head 00000000 Count 005eaf35
[36936.795626] NETDEV WATCHDOG: eth1: transmit timed out
[36936.795633] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=2036.
[37134.660593] NETDEV WATCHDOG: eth1: transmit timed out
[37134.660600] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=46288.
[37144.653773] NETDEV WATCHDOG: eth1: transmit timed out

Take a look at the lag between [  159.931681] [36590.032279].

After killing xorg with kill -9 there are new entris in dmesg:

[36936.795626] NETDEV WATCHDOG: eth1: transmit timed out
[36936.795633] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=2036.
[37134.660593] NETDEV WATCHDOG: eth1: transmit timed out
[37134.660600] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=46288.
[37144.653773] NETDEV WATCHDOG: eth1: transmit timed out
[37144.653780] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=1289.
[37450.445078] NETDEV WATCHDOG: eth1: transmit timed out
[37450.445086] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=74039.
[37460.438258] NETDEV WATCHDOG: eth1: transmit timed out
[37460.438265] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=1542.
[37722.259577] NETDEV WATCHDOG: eth1: transmit timed out
[37722.259584] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=63042.
[37741.246615] NETDEV WATCHDOG: eth1: transmit timed out
[37741.246622] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=3294.
[38081.014820] NETDEV WATCHDOG: eth1: transmit timed out
[38081.014827] eth1: Tx timed out, excess collisions. TSR=0x1e, ISR=0x8, t=2045.
[38150.386362] NVRM: RmInitAdapter failed! (0x12:0x2b:1544)
[38150.386371] NVRM: rm_init_adapter(0) failed

I tried to restart X but it doesn't work:

(II) NVIDIA(0): Support for GLX with the Damage and Composite X extensions is
(II) NVIDIA(0):     enabled.
(EE) NVIDIA(0): The NVIDIA kernel module does not appear to be receiving
(EE) NVIDIA(0):     interrupts generated by the NVIDIA graphics device
(EE) NVIDIA(0):     PCI:5:0:0.  Please see Chapter 5: Common Problems in the
(EE) NVIDIA(0):     README for additional information.
(EE) NVIDIA(0): Failed to initialize the NVIDIA graphics device!
(II) UnloadModule: "nvidia"

Chapter5 says something about irg Sharing. Perhaps the root of this problem is a problem between the Network Card an the NVIDIA Adapter? Perhaps somebody has an Idea how I can check this? Compare lspci and ifconfig perhaps?