Update: doesn't seem to be specific to KMS, as I think I've been having the same problem with KMS disabled. Also, here are some messages I've been getting, though I'm not sure if they're relevant: 

May 30 15:04:48 pia kernel: [   36.520173] integrated sync not supported
May 30 15:04:48 pia kernel: [   36.804170] integrated sync not supported
May 30 15:04:48 pia kernel: [   37.080175] integrated sync not supported
May 30 15:04:52 pia kernel: [   41.136237] integrated sync not supported
May 30 15:05:26 pia kernel: [   74.736338] gvfs-gdu-volume[4741]: segfault at c ip 0065ffda sp bfdf6c10 error 4 in libgdu.so.0.0.0[657000+22000]
May 30 15:05:38 pia kernel: [   87.120151] gvfs-gdu-volume[5034]: segfault at c ip 0050bfda sp bfe725b0 error 4 in libgdu.so.0.0.0[503000+22000]
May 30 15:05:44 pia kernel: [   92.772304] mixer_applet2[4058]: segfault at aaaaaaac ip 00230be2 sp bfaf1a90 error 4 in libglib-2.0.so.0.2100.0[20d000+c3000]
May 30 15:05:46 pia kernel: [   94.956223] integrated sync not supported
May 30 15:05:46 pia kernel: [   95.240227] integrated sync not supported
May 30 15:05:48 pia kernel: [   96.452239] integrated sync not supported
May 30 15:05:51 pia pulseaudio[5334]: pid.c: Stale PID file, overwriting.
May 30 15:05:53 pia kernel: [  101.876205] integrated sync not supported
May 30 15:05:53 pia kernel: [  102.156170] integrated sync not supported
May 30 15:05:54 pia kernel: [  102.436207] integrated sync not supported
May 30 15:05:57 pia kernel: [  105.448172] integrated sync not supported
May 30 15:06:14 pia kernel: [  123.376261] integrated sync not supported
May 30 15:06:15 pia kernel: [  123.652237] integrated sync not supported
May 30 15:06:16 pia kernel: [  124.836227] integrated sync not supported
May 30 15:06:20 pia pulseaudio[5981]: pid.c: Stale PID file, overwriting.
May 30 15:06:22 pia kernel: [  130.436180] integrated sync not supported
May 30 15:06:22 pia kernel: [  130.716174] integrated sync not supported
May 30 15:06:22 pia kernel: [  130.996176] integrated sync not supported
May 30 15:06:25 pia kernel: [  134.256363] integrated sync not supported
May 30 15:06:30 pia bonobo-activation-server (jon-6227): could not associate with desktop session: Failed to connect to socket /tmp/dbus-A933wHNT6o: Connection refused
May 30 15:07:17 pia kernel: [  185.424992] gvfs-gdu-volume[7108]: segfault at c ip 006ccfda sp bfe12650 error 4 in libgdu.so.0.0.0[6c4000+22000]
May 30 15:07:23 pia kernel: [  192.085630] mixer_applet2[6171]: segfault at aaaaaaac ip 0039dbe2 sp bf87d150 error 4 in libglib-2.0.so.0.2100.0[37a000+c3000]
May 30 15:07:25 pia kernel: [  194.144226] integrated sync not supported
May 30 15:07:26 pia kernel: [  194.420241] integrated sync not supported
May 30 15:07:27 pia kernel: [  195.604226] integrated sync not supported
May 30 15:07:31 pia pulseaudio[7427]: pid.c: Stale PID file, overwriting.
May 30 15:07:33 pia kernel: [  201.572191] integrated sync not supported
May 30 15:07:33 pia kernel: [  201.852172] integrated sync not supported
May 30 15:07:33 pia kernel: [  202.136270] integrated sync not supported
May 30 15:07:36 pia kernel: [  205.316192] integrated sync not supported
May 30 15:07:57 pia kernel: [  226.235963] gvfs-gdu-volume[7681]: segfault at c ip 00544fda sp bf903440 error 4 in libgdu.so.0.0.0[53c000+22000]