This has been covered before but I thought it might be useful to confirm the fix and add a few details.

Symptom: At idle, Speedometer needle jumps indicating various speeds while car is stationary and immediately returns to zero. It is an intermittent problem. Car could be in park or drive. Sometimes but not always, when the speedometer needle jumps the rpm will increase without touching the gas. If you are in drive with your foot on the brake the car may try to surge ahead. Battery voltage may appear normal and alternator appears to work normally. You may or may not have the battery indicator lit on the dash. Mostly, no codes are ever set when the fault appears however at random the following may be set: P0562, P0944, P1790, P0218. I suspect these codes are thrown at random and it is likely virtually any code could be thrown.

FAULT FOUND: The alternator was defective. Mine appeared to be functioning normally and could put out the full rated amps under load. It did pass when checked with one professional tester, however it failed on another which correctly indicated a diode problem. The alternator was rebuilt & problem fixed. This fix for this specific issue has been reported by many others.

DETAILS: In my case there was a post that connects to one side of the diode pack where it is fused to the casting which had cracked & broken free. It was almost invisible to the eye. Some contact remained however and it showed variable resistance. I can not say precisely how this effected the speedometer but I suspect this is simply incidental, it could have resulted in any goofy behaviour with any of the computer controlled systems. My guess is the poor and intermittent contact allowed the alternator to keep putting out a charge but at the same time was causing some sort of noise/feedback or high freq AC signal to the computer which was scrambling the speedometer circuit.