Went out to start car a while back and it was clear that engine not under computer control (gauges not working, engine fan on right away, poor idle, Service Engine Soon light on). Took it to local Chevy dealer (yeah, I know, but don't know who to take it to here in central Jersey!) and they said I needed a new ECM module. Gave them the OK to replace ($300 or so) then they called back next day and said that I needed a new chip and the one I had wasn't available (and my "old" ECM had already been sent out). I had an old one that was in a rear storage area when I bought the car and this seemed to fix the problem except that the SES light still on and the shift light comes on at highway speeds but I have an AT transmission!!! They said that I needed a new EGR valve, but I know how to replace that so I did. The SES light is off now but shift light comes on at highway speeds and the engine seems to miss or "stutter" occasionally (or is it the tranny?)at cruising speeds, but engine roars if I punch it and shift light goes off once I gas it or drop below bout 1800 RPM.

Any ideas what's going on here?

 

Your old stock chip and the storage chip you found may not be the same deal. Check/compare the numbers on the main proms. There are many different stock programs on these chips for the same year car. Eprom/calpack don't usually go bad unless you hook the battery cables up backwards. If the stock chip was in fact bad then chances are your ECM was good so they were just part swapping stuff for you until they stumbled onto the problem. Take it back to the dealer and have them resolve the issue.