1992 Chevy Silverado 2500 4X4.200,000 mi. Question:

My speedometer is giving me fits. It works sometimes. Seems like when the engine warms up, it starts to fluxuate. Up and down. Works then dosen't.I've had the dashboard out and is not cable run. Just plugs in. What is the cause of this little dilema? Speed sensor perhaps if so, is it located on the transfer case? Or where is it. All other gauges in the dash work fine. Just the speedometer. Also, after the truck is warm and has been running for awhile, when I pull up to a stoplight and sit there for approx.45 sec give or take. It surges R.P. M.'S go down and it stalls, sometimes. Not all the time. Idle speed sensor bad.I don't wanna set the idle up cause it seems to idle just fine. I have performed a tune-up on it. Changed the spark plugs, distributor cap and rotor, fuel filter, air filter, plug wires. Ect.I am at wits end here. HELP please!