90 240sx speedometer is surging, please help!

[newboyspo]

Hi,When I first start up my car the speedometer surges up and down from 0 to whatever the speed is that I am traveling. After I have driven for 10 or 15 min. it starts working again until I park the car for awhile, then it starts surging again..Hiting the dash, tapping the face of the speedometer does not help, I checked out the wiring on the sensors on the transmission (all are connected and not shorting...could it be the one on the very back of the transmission? gear drive converted to electrical signal?? (just guessing on this)

Any ideas to point me in the right place to start, i.e. behind the speedometer, tyranny sensor, wiring?

Thanks

[180fan]

Speedo sensor's at the back of the transmission. Can't miss it. Sticks out like a sore thumb.