96 Lumina speedometer erratic at idle

Hi all,
The Speedometer seem to show the corect speed while driving, however, when I stop (or am almost at a dead stop) for a light or such, the speedometer goes crazy. It will jump up and down the scale until I start rolling again. It seems the if I am below 5 MPH, it acts up. Also, the engine will "die" sometimes sitting at a light. I restart and take off with no problems. This seemed to start happening when the speedometer started to act up.

I checked the wiring with a meter to the VSS and seems good. What kind of signal should I look for out of the VSS with the car in motion?

Any thoughts????
Thanks!

I have a little more info....

Now, the speedometer acts up constantly, even while driving. I did notice the following:

When going aproximately 55mph, I locked in the cruise and it works normally. Speedometer will oscillate up to 110MPH and back down constantly now.

When the speedometer is pegged on the high side, sometimes the "oil level" light will come on and the engine dies for a brief second, then the light turns off again and the engine runs normally.

This happened only a few times: When at a stop light, sometimes the speedometer climbs past 100MPH when I push on the brake pedal, and go back to zero when I release the pedal.

Anyboy have ideas????

Check voltage out put at the alternator, make sure there is no ac voltage also.

Thanks maxwedge, I think you are "right on the money"!!!

I was going to post this, and saw your suggestion just now.

Last night I took the instrument cluster out and opened it up to clean it out real good, and see if there is any corroded solder joints. I found nothing that stood out. Put it back together, and the same symptoms.

However, in my haste, I DID NOT disconnect the battery!! ( I know......)

So, thinking about it later, I went to disconnnect the battery, thinking that maybe "rebooting" the PCM would do something. I found that the negative terminal fasterner was cross-threaded just about a half a turn into the battery and seemed tight. When I re-connected it, I made sure that it went on like it should.

That seemed to fix it!!!! Drove about 50 miles on it between last night and this morning to work and not a single incident.

I know that the battery acts sort of like a filter for DC, reducing any noise (ripple) from the alternator (or AC). I also notice that the alternator "growl" I had for a few months has gone away.

Again, thanks Maxwedge for your input, and you were right.

Funny thing though, I would have thought I would have had trouble with the starter being that the cable was not securely fastened.