Strange voltmeter readings

[Vic Norman]

The voltmeter on my 1991 Audi 100 normally registers a little over 12 volts
when the engine is running. On occasion, however, the reading will drop
considerably. Lately, it's been all the way down at 8 volts at times.
Turning headlights etc on or off doesn't change the reading. Nor does the
low reading affect the car, it starts and runs fine,  so I'm sure the
battery isn't really down to 8 volts.
Anyone have any suggestions as to whether I should be worried or just ignore
the damn thing?

Thanks.