Check Engine Light and Reduced Power Light still comes on after tune up maint... on 2004 GMC Envoy

I've done a basic tune-up ,oil change,spark plugs,fuel filter,air filter,also changed the thermostat and cooling sensor @ a garage. Cleared the code. Ran ok for a day. Now both lights are on again! When I start vehicle up in the morning it warms up, then idles down greatly, reduced power light pops on and stays on..I turn it off and start it up again..same thing! When I put it in gear it crawls, then stalls..does this cycle over and over for about 30min. Need help with issue..

Asked by for the 2004 GMC Envoy
3 answers
What was the code? Or did you just ask to have it turned off?
is your defrost on? clean your throttle body
I had the same problem. My mechanic could not find the issue, took it to the dealership, it was a computer issue. Very expensive part, the dealer was able to find a used computer part for $1000.00, but it solved the problem.