I had this exact problem with my Indivision, Apollo 1260 and Escom (HP Lisa) 1D4 board. It was obviously heat related - gently pressing down on the Indi eliminated the problem entirely. My socket was already factory modified. I ended up adding a small quiet fan above the Indivision, problem solved. Another idea would be to add some material that lightly presses down on the unit with the case closed - naturally just in the center of Lisa so it doesn't trap heat etc.
