Question: Will it do it with your expansions/cpu card disconnected? A poor contact somewhere can cause similar symptoms, and often it will only show up after the machine is warm. Might want to re-seat the SIMMs (and any other socketed chips), clean contacts, etc... and please forgive me if you've already done so.
I had a similar problem recently on an A2K, turned out that the Agnus (seated within a MegaChip) was causing all problems... :/
