Okay,
I've got three maybe-helpfull-ideas
1) If you have a 1MB card, check if it is a) connected properly and b) what happens when you remove it. I had an A500 a few years back with the exact same symptoms, turns out the memory card had died somehow.
2) Stupid one for SMD, but anyway, see if the Kickstart is still securely in place and try to find if any of the chips are 'loose'.
3) Powersupplies for the Amiga (except big-box ones) are notoriously bad, try a different one if at all possible, or since you are a technician, check if the named voltages and amp rating actually get delivered to the Amiga. You never know!
The disclaimer to this one is that I'm just following what works for me and I am in no way a qualified Amiga repair man. Take it as you will :-)
