Guru/Error 8000 0004 is ILLEGAL INSTRUCTION
Could be one of the ROMS is failing and causing this error if it's intermittent (doubtful though)
or a loose ROM chip on the MB, or on the trapdoor expansion board if you have one, but then it could something as simple as the PSU or HD failing...

To trace such problems I generally remove all add ons from the A1200 and start with a bare system (ie: just the floppy drive installed), I know this may seem a pain and time consuming but it usually narrows the problem down and save you a lot of mucking about in the long run...
