>I'm still wondering if it's the memory itself. At $30 for 256, I'm half tempted to replace it and see if that helps.
For a free test, you could always remove the extra memory (get back to 128Mb) and leave it at that for 2 or 3 days. If the problem doesn't reappear, try swapping memory with the one you just removed.
Sylvain Demers