Cheers for the answer. I do indeed have 2 different sticks of memory, so you could be on the right track. But I have had them in for over 6 months, and only recently have the problems started...
Still, I will borrow some memory and do some tests.
Also it does not seem to be when gaming. Lately I have only been playing Diablo LoD, and have no trouble whatsoever there.
It *seems* to be when I open a few different windows, but it has happened when I have only a textpad open.