2003 GMC Sierra info please

Ok first off check engine light comes on for fuel to rich probelly the k&n cold air intake have a throttle body spacer also dident think to much about it ran fine and strong like always, then it started getting real weak check code again o2 sensors so I replaced all 4 of them put my factroy air intake back on with a new filter and changed the fuel filter, engine light went off ideals fine but still had no power but sometimes it hits a burst of power for about 5 seconds then falls on its nose again, and also noticed it want set in gear (manuel) keeps inchin back on a very slight grade thought maybe losing compression? Any info or something else to try would be great thanks.