1992 eighty-eight HIGH VOLTS Problem

I have a 1992 Oldsmobile Royale LS with a 3.8L engine the dash is all computerized.
About a month ago, my car started reading HIGH VOLTS on it. I look on the dash guage and it's getting 20 volts from the lines. I've replaced the alternator 3 times and it still does the same thing. I've cleaned the battery cables off and this doesn't work. While running, it will Idle around 1000-2100 RPM on the digital tacometer. Also when pushing on the gas with this event occuring I have to press down on the gas more then I would just to get the same power. This problem happens about 80% of the time I start the car, then it will refuse to start up again afterward until I unhook the battery cables and re-attach them after a wait.

I've cleaned the two grounds that are in the engine bay and nothing happens. What next should I do? No one seems to be able to help solve this problem. If I should check a cable, which one and where? if I should replace a part, which one? Lastly, I'm looking for the ground under the car near the transmission, can anyone tell me where this one is, and if this is a possible fix to the problem?

Welcome to AF, have you put a voltmeter on the alternator to check the actual voltage, checked engine to chassis grounds?

I haven't been able to get ahold of a voltmeter, but 3 different alternators producing the same result must either be REALLY bad luck or something else. As for the grounds, I have checked the ones in the engine bay and am seeking the ones under the car, but don't know their relative location, assistance in that would help me if you would like to part with that info :). So is there any other things? The last resort would be completely replacing the computer systems, but I don't want that big bill if it's something else. So what else could be causing this?