Good luck finding a solution to these issues. I'm an ex-1200 owner and even in the late nineties they seemed a lot more flakey once I started going beyond a memory upgrade and a 2.5" HD (tjen the advice was always get an up-rated power suppy).
What's deemed to be the most reliable, stable, dependable classic Amiga workhorse available? I had a 4090 and that seemed worse still...could never trust it for important work. Would eg. a 500+ be more solid/stable than a 1200?