1989 Ford Probe charging system problems

1989 Ford Probe LX mileage: 108,000. I have a question regarding my car's charging system. It has a history of charging system problems. The system completely failed about three years ago, I ended up replacing the battery, alternator and power distributor. Everything was fine until about a year ago when the system quit again, I had it checked and it was the alternator again. I replaced it myself and it was fine again until now. However, now it has some occasional problems with overcharging. I have not had any problems with light bulbs or fuses burning out. However, a couple times a week the voltmeter will read slightly above normal. There were a few squeaks in the belt, after which I tightened it and although the noise is mostly gone, the same problem with overcharging still exists. I have also noticed that the car seems to lose some power when it is doing this, but the acceleration will pick up again once the voltmeter is reading a normal charge. Do you have any ideas on what might be causing this?