When PCM's Go Bad
 

I threw a 1683 code which I suspected was a failing PCM and the Chevy dealer confirmed it. After they replaced the PCM (under the Federal 8yr/80K mile emissions components warranty), programmed it to my VIN, and returned it to me, I noticed that the idle was only 400 rpm and at 60mph was 1300 rpm instead of the normal 1800. Plus, my Hypertech programmer did not recognize the vehicle at all!
When I called them and said I suspected that the new PCM had been programmed to the wrong VIN, they acted like I was off base, but said to bring it in. They told me everything seemed normal, nothing amiss, but I got with the shop foreman and forced him on the lower rpm issue. After we hooked-up a Tech2 scanner, drove it and verified the discrepency, they tried to reprogram it and realized the VIN was not downloading into the PCM. The solution was to manually enter the VIN into the program and - voila! Blazer runs like new (except I need to replace the distributor cap & rotor and plug wires).
The lesson they learned is that nothing is foolproof and that factory computerized electronics can be wrong.

Glad you got them to take a closer look over at the dealer. I know sometimes it's like pulling teeth to get them to admit another perspective on a particular problem.

Cheers.