To me it sounds like a faulty PSU... I also had an A500 PSU connected to my A1200 once but I connected too much to the machine anyway and got unregular reboots without warning even after removing most of the stuff I had connected. :-(
So I opened the PSU, saved the entire power cable that goes to the Amiga and connected that to an AT 230W PSU instead and the problem was solved. Later the AT PSU got stripped from it's shield and the components was put in a custom perspex box and that looks mighty fine. :-D
