I have a Macintosh IIci, running 7.6 with a special Mac to VGA adapter so I can use it on MultiSync (modern) screens. The Mac starts up fine but after about 5-10 minutes the Mac turns itself off, when you turn it on again in a half a minute the machine turns off. Then if you try and turn it on again it will only be on for a few moments. I'm guessing I need a new power supply. I just want to confirm this before I buy anything. Thanks in advance!