HI!
I've the same problem with an A2000 rev 4.5 of my Amiga collection. I'm looking for the solution, but I think that the problems comes for a bad track or soldered point near the processor (pins) or battery (in a2k there are very near). In my a2k the battery lost acid and two tracks were broken. If I found the problem i'll post it.
RRete
A500(3),A500+, A2000 rev6, A3000, A4000 060, CDTV, A600, A1200 HD.