I am experiencing frequent crashing, not apparently linked to any particular keyboard action. System Log File Viewer shows that immediately before each crash Pulse Audio was trying to do something; this is a typical example:
Aug 19 17:50:48 henry-desktop kernel: [ 16.645113] type=1505 audit(1282236648.852:8): operation="profile_replace" pid=642 name="/usr/lib/connman/scripts/dhclient-script"
Aug 19 17:50:48 henry-desktop kernel: [ 16.735389] type=1505 audit(1282236648.940:9): operation="profile_load" pid=708 name="/usr/bin/evince"
Aug 19 17:50:49 henry-desktop kernel: [ 16.827981] type=1505 audit(1282236649.032:10): operation="profile_load" pid=708 name="/usr/bin/evince-previewer"
Aug 19 17:50:49 henry-desktop kernel: [ 16.849772] type=1505 audit(1282236649.056:11): operation="profile_load" pid=708 name="/usr/bin/evince-thumbnailer"
Aug 19 17:50:49 henry-desktop kernel: [ 17.376171] apm: BIOS not found.
Aug 19 17:50:58 henry-desktop pulseaudio: ratelimit.c: 2 events suppressed
Aug 19 17:51:21 henry-desktop pulseaudio: ratelimit.c: 4 events suppressed
Aug 19 17:51:27 henry-desktop pulseaudio: ratelimit.c: 1 events suppressed
Above are the current latest entries, before the next crash. I have reinstalled Ubuntu and updated and upgraded it to 10.4. In an effort to solve the problem prior to that, I deleted all references to Pulseaudio in Synaptic with the apparent result that sometime later the login screen would not work which why I had to resort to reinstalling Ubuntu. The symptom has carried over from my previous machine to this one; the only common hardware is the second hard drive which contains my files, no OS. The first drive has both Win XP and Ubuntu in different partitions. Of possible relevance may be that although number lock is set to on in the BIOS, I have to set it myself after starting. One final clue, to restart the frozen machine I have been holding Control Atl Delete for several seconds with apparent success.