At first consideration, it sounds like a bad voltage regulator -- the battery voltage fluctuating could certainly do this, which is what the voltage regulator (embedded in your alternator) is supposed to prevent as well as keep your battery at the proper level to prevent overcharging. It certainly sounds like a serious loss of regulation, however brief, because it takes a lot of increase in voltage to make the wipers go noticeably faster.

I suppose you don't have voltmeter added to your dash instruments, but do you have a handheld DVOM that you could hook up to your fuse panel and watch (while keeping your eye on the road :-) as you drive around? Or, some auto parts places have a voltmeter built into a plug that goes into your cigarette lighter?

Have you checked your battery's electrolyte level. I'd suspect that your battery is intermittently overcharging and you're low on fluid -- this would be the result, not the cause, of the problem, however. But it would support the hypothesis.

However, the fault could be, instead of the voltage regulator, some high amperage accessory that's intermittently drawing a lot of current. You might be confusing low voltage (because of the faulty accessory) and its effects as the "normal" state, whereas the problem is not high voltage but actually low voltage. Possibilities include the rear window defroster, the door locks (do you hear any clicking) which have a relay that's notorious for going bad, an auxiliary electric radiator fan (do you have one in your model year, I'm not familiar with '90s), and even your A/C compressor.

Another possibility is the wiring between battery and alternator, which could be affecting how well the regulator regulates.

Anyway, for whatever reason, it sure sounds like variations in your system voltage. Go from there.