Sounds like the PSU. How's it causing problems with the Monitor? Maybe the PSU is still providing some power even when switched off. Not to mention, it's probably providing an unstable power signal.
I wouldn't use that PSU anymore and start looking for another. Ever think of towering your A1200?
P.S. Do you or anyone you know have a Electronic Multi Meter? You could check the voltages and amps on the power signals going in and out of the PSU. Then you could see if the proper voltages are being supplies and if there are any wild variations.
