C64 questions

I've got an application that depends on high precision timing that I originally wrote for the C64. I decided to use it to acquire more data and had very bizarre behavior from the C64 in that at first the standard screen didn't come up but then it did and the letters faded and the C64 was unresponsive to keyboard input or just showed garbage characters. I happen to have a spare C64 and it demonstrated exactly the same behavior. The 5 V supply reads 4.94 V under load and has little ripple. There is 9 VAC coming out of the power adapter and I get +5 V and +12 V at the outputs of the internal C64 regulators. There seems to be little ripple on these supplies also (about 20 mv) but this check was done with the frequency measuring setting of my DVM so I don't know if there are short HV spikes on any of the supplies (have to try to remember where I put my scope after the last move). The C64 that I want to use is 27 years old and worked just fine when I last fired it up about 10 years ago. Anyone have any experience with this type of behavior? Is it worthwhile to replace the internal power supply electrolytics or should I build a new power supply for it? Have lots of 5 VDC supplies but don't seem to have a 9 V transformer in my junk box. Boris Gimbarzevsky