2007 GMC Sierra

Engine light was on, had it checked at dealer, first recommendation on dealer computer to clean throttle body, I did this myself (an easy task) engine light went off, engine running smoother and much better throttle response. Only problem now is it idles at 1800 rpm, took it to dealer and they reset the computers idle, worked for a short time then back to 1800 rpm. I sprayed carb cleaner around gaskets to check for a vaccum leak, nothing. I took the throttle body back off and used a little gasket sealant on gasket just to make sure, it does have an airraid throttle body spacer on it but put it on @ 6,000 miles, can’t see that making a difference after 50,000 miles. Engine light is off and dealer finds no codes on computer.