[bpodner]

There was a tsb for GM instrument clusters a while back, don't know if it's still valid. The speedo did the same thing on my 03 Z71 Suburban. After going round and round with the dealer, and reporting it to the NTSB, the tsb finally came out. The work around is to cycle the ignition key on/off, this brings the speedo down to 0 in 5 mph increments. There used to be a guy on ebay that rebuilds the clusters for a lot cheaper than GM, try a search he had a lot of good feedback. The stepper motors are what goes bad.
By the way, that vehicle was the last GM vehicle I ever bought, a $50000 POS.