When i am driving at freeway speeds and floor it to pass up a car my car does not downshift to third . at first i thought it was but when i pull the shift lever down into third from drive i feel the difference.. i did have a new pcm installed and programmed . could there of have been a programming mistake or can it be from a faulty vaccum line. ANY SUGGESTINGS WOULD HELP. i also barely had a head gasket job done .. 1995 buick supercharged 131,000 miles.