Do you know if :-
* The busboard,
* The accelerator,
* The memory.
All work ok ?
Try using different combinations to see if one of them is faulty.
I had a similar problem with an old A1200 board when adding an accelerator or extra memory, the only thing that would fix it was the motherboard fix that Eyetech do. Is the motherboard a 2B or 1D4 revision board (there are some others that play up too)
