Voltage problem

My 94 PA suddenly developed a strange behavior. The voltage gauge swings unmotivated between the two red markers. Normally, it used to be just below the high red zone. Actron CP9150 now reads voltages between 11V and 13.6V. The car starts OK, idles OK, although at idle, the voltage is in the lower area mostly.
When I drive it, the engine runs fine. But while driving, the voltage fluctuates with no specific pattern. It can be OK for several minutes and then start to swing at an interval of a few seconds. When it does that, the engine noise (or better: noise from the engine compartment, since the rpm stays the same) changes also. It sounds to me like the alternator changes its speed or something like that?! Could a slipping belt cause this?

Any hints are greatly appreciated.

Check the alternator output with an avr tester, particulary with an electrical load on it., look for bad gd/pos connections also.