Chevrolet Repair: Failed speedo, speedometer needle, salvage yard

Question
1991 Buick Lesabre (hopefully similar to Chevy)  
Speedometer needle jumps occasionally, inop most of the time.  There is no check engine light, the transmission sifts normally, the cruise control works normally. (Eliminates VSS I believe)  I have changed the inst. cluster with an identical unit from a salvage yard resulting in no change.  I understand the PCM is the "middle man" in the speedo circuit.  Is there a shade tree technique to check the PCM short of replacement or dealership diagnosis?  Is there a standard location for instrument panel grounds that I might check?  What color wires go from the PCM to the speedo-so they can be checked for damage?  My entire auto repair experience can pretty well be described as "if it has happened to my car, it has probably happened to MANY others.  Thanks for whatever info you might be able to provide.

Answer
Art,

I picked up this question out of the pool. It was not sent to me. Before working for Chevy I worked at Oldsmobile for 10 years and Buick for 2 years. My past experience with this complaint most always was a defective ECM. In fact in my own car, a 90 OLDS 88 which is an H-body, same as a LeSabre I have put 2 Ecm's in over the last 10 years for intermittent speedo. I'm keeping the car for my Son to drive and Guess what...the speedo is inop again, only on humid days. I know it's the ECM but have not got around to replace it. The ECM is these vehicles have been updated a few times.

I would suspect the ECM is the cause of your problem. You can try tapping on the ECM with the engine running. If the engine stalls or stumbles or the speedo jumps...replace the ECM for sure.