I have finally come to the conclusion that swap file thrashing due to excessive memory use by one problem program is the cause.
My browser of choice is Mozilla Seamonkey which is the remote descendant of Netscape Navigator. It has a problem in that its memory usage creeps upward and eventually there is not enough memory for some action in Seamonkey to complete. If I have not used F11 to go full screen I can sometimes get into System Monitor and kill Seamonkey but if I have already gone to full screen mode I have to power the machine off and reboot.
I have 4 GIB on my laptop of which 3.7GiB is available but use of more than 3.0GiB of it always causes slow responses, high HDD activity and intermittent operator control of mouse and keyboard. Sometimes sympoms appear with as low as 2.9GiB used.
Whether this a bug in seamonkey or whether its memory needs would level off if I had more memory available I do not know. As soon as I transfer my home directory to my larger 16GiB computer I will test this.
Now how do I mark this discussion as closed.