In my 1986 nissan 2wd pickup, the tachometer will start making some wild fluctuations after about 5 or 10 minutes of driving. Occasionally it will work fine. Having the lights on seems to worsen the problem.
Does the tachometer wire attach to the distributor/ cap? I tried cleaning a wire connection there, with no luck.

The tach. in my '93 quarter ton also spikes at different times. Sometimes it stays at a very high rpm for a few seconds while other times it spikes and then immedialtely goes back to the 'real' rpm. It doesn't happen very often; maybe once or twice a week. I guess I've just come to live with it as a quirk in the truck. I know this doesn't help you solve your problem, but I figured I'd let you know you're not alone.