speedometer screws up

ok, heres the thing - my speedometer is good most of the time, but when im getting on to the highway (65mph) i dont have perfect shifting from rushy traffic n such (manual car) so my speedometer reads good til about a mile after i get on, if i screw up a lil shifting (but not snubbed quite) n then the speedometer goes up about 5mph then down below what im goin about 5 mph n then goes back up to what im going, then goes down to like 30 ish n stays there til im done driving
if i let the car sit off for like an hour or so, it comes back if i drive. so my speedometer is good most of the time except for if i do crappy shifting n then it comes back after sitting for a while.. any ideas what this could b and how i could fix it.??
(its only the speedometer, i know my speed pretty much by knowing the rpm/speed ratio, and nothing else on the cluster pannel goes)
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and by the way, its a v6 99mercury cougar

These damn electrical cars...well im havin trouble with my trip/mpg/temp indicator...scrolling through the options on its own..without me hitting the select button...weird...but i'm startin to think its a short...and that just might b ur problem...just wingin it here...good luck