I had a similar problem in the past whereby I ended up having to reseat all my boards. I have not had the same problem since, but am instead getting random lock-ups of the Amiga.
This seems is something that started to occur when I bought an Apollo 68060 for my A1200, but since I have bought the Mediator + 32Mb of RAM the problem has gotten worse, especially when surfing the web.
I have since rebuilt my A1200 twice this year. If anyone has any other ideas I am open to options.
The last suggestion I received from the newsgroups was to remove everything and plug in the expansions one-by-one which I want to avoid if poss.
Regds
Jono