I have similar problems after upgrading from Hardy to Lucid. Sound always stops after playing movies or using other applications for a few seconds/minutes regardless of the application used (I broke the sound with mplayer, vlc, totem and even the Multimedia System Selector app sound test).

Once broken, I can get sound back if I restart pulseaudio with:
$ pulseaudio --kill; pulseaudio --start

Sound always worked well in Hardy except for some glitches regarding sound control.

Each time sound breaks, I see this in syslog:

Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: snd_pcm_delay() returned a value that is exceptionally large: 8574853690509206496 bytes (360287948221 ms).
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: Most likely this is a bug in the ALSA driver 'snd_hda_intel'. Please report this issue to the ALSA developers.
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: snd_pcm_dump():
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: Soft volume PCM
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: Control: PCM Playback Volume
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: min_dB: -51
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: max_dB: 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: resolution: 256
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: Its setup is:
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   stream       : CAPTURE
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   access       : MMAP_INTERLEAVED
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   format       : S16_LE
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   subformat    : STD
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   channels     : 2
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   rate         : 48000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   exact rate   : 48000 (48000/1)
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   msbits       : 16
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   buffer_size  : 96000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_size  : 48000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_time  : 1000000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   tstamp_mode  : ENABLE
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_step  : 1
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   avail_min    : 48000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_event : 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   start_threshold  : -1
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   stop_threshold   : 6755399441055744000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   silence_threshold: 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   silence_size : 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   boundary     : 6755399441055744000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: Slave: Hardware PCM card 0 'HDA VIA VT82xx' device 0 subdevice 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c: Its setup is:
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   stream       : CAPTURE
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   access       : MMAP_INTERLEAVED
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   format       : S16_LE
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   subformat    : STD
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   channels     : 2
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   rate         : 48000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   exact rate   : 48000 (48000/1)
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   msbits       : 16
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   buffer_size  : 96000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_size  : 48000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_time  : 1000000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   tstamp_mode  : ENABLE
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_step  : 1
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   avail_min    : 48000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   period_event : 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   start_threshold  : -1
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   stop_threshold   : 6755399441055744000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   silence_threshold: 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   silence_size : 0
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   boundary     : 6755399441055744000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   appl_ptr     : 2880000
Apr 27 02:55:15 dermoth pulseaudio[32250]: alsa-util.c:   hw_ptr       : 1825528

As for the hardware I run on, you can see attachments for this bug which I reported today...
https://bugs.launchpad.net/bugs/570454

dmesg doesn't show anything new besides these two pulseaudio segfaults which seems to have occurred during two of the numerous times I broke the sound while testing:
[60980.859078] pulseaudio[27646]: segfault at 7fe6f35df1a0 ip 00007fe6f35df1a0 sp 00007fffffb95518 error 14 in pulse-shm-2848538534[7fe6f3c03000+4001000]
[65109.280919] pulseaudio[874]: segfault at 7fd0f20fc1f0 ip 00007fd0f20fc1f0 sp 00007fffcca81438 error 14 in pulse-shm-4084731526[7fd0f2720000+4001000]

Please let me know if you need more info.