When I run my engine in park at lightly high RPM it has a regular fluctuation, it does the same at idle but not so regular. I installed a new EGR and EGR valve, I checked the MAP, IAC, TPS and their good. The O2S didnt check out, could it cause this? What else should I check? I have a '90 GMC Sierra with a 5.7 TBI with a automatic tranny.