86 coupe losing power

[Mark Donohoe]

I have recently replaced the battery and alternator. Due to rarely
driving the car, I will start it up to run for a while.  Yesterday,
started it, it ran for about 15 minutes and then I noticed the engine
starting to struggle and the digital dash had gone blank.  When I revved
the engine, the dash came back on and I noticed the volt guage increase
at a rapid pace up to the normal level.  I took the car out for a spin
and it runs fine. Since the car sits most of the time, I don't leave the
battery connected because the person I bought the car from suspected a
short was draining the battery.  I thought he just had an old battery,
dead from no use.  Maybe he was right after all.  What would cause power
to drain down while the car is running?

Thanks,
Mark D