93 MIATAS RPM GOES UP AND DOWN;BUT NOT ALL THE TIME

any trouble codes? the Check engine light come on? you did not say it was a cold engine...but not all the time means what? does it when it's warmed up? want to know more about your particular problem, sounds like an old manual distributor, but I wouldn't think they still used these on the '93 models. A centrifugal avance mechanism in the distributor will give you somethin' like this-

Ah. My 5.0 does this sometimes... Usually only when its hot and in park. Check engine light blinks sometimes en the highway... I havent even read the codes (Lazy me) but I just assumed it was probably something with the EGR valve and I was dreading taking off the exhaust tube bolts >_< Bob brings up a great point though... An old distributor will definitely have these symptoms.

Is your A/C on? If so your compressor may be kicking on and off...