ok so i have a 2001 silver ado with the 4.8 150k i bought it used a year ago. i have a p0121 code throttle position sensor out of range. i replaced the tps. code came back. found my throttle body was all gunked up. so i cleaned it all up. reset codes. and it came back. checked and tightened all the electrical connections and grounds on truck and checked wiring harness.. looks good reset. came back. checked voltages at tps. battery ground to output on fps 5volts. battery positive to tps ground 12v. looks rite per book.. reset came back. read up on how it works. ended up replacing maf sensor. reset codes. drove and it returned. had truck scanned found map sensor off by 1 point. replaced and code returned. replaced all spark plugs. i did find they were gapped at 36, book calls for 60. set all to 60 and installed. they are the iridium type. reset codes and they returned. checked for air leaks checked all hoses and connections, all looks good. reset. returned. replace tps again just in case. reset and returned. replaced computer with reprogramed one with current gm program. code returns. pulled and checked air idle valve. broke it. replaced it. reset and code returned. the idle seems a little erratic after replacing air idle valve. runs fine. this whole time the truck has run fine. seems to shift fine, a little ruff some times but no real problems. i will reset codes. it will drive fine for 10-20 miles and it seem every once in a wile i will be at a stop light and when i pull off the line i will have no throttle response. it will pick up speed slowly and runs fine and after that is when the code returns. the truck had the code when i got it so i can't jude it for normal operation, i would seem to no be very powerful. but it is a 4.8 my last one was a5.7. . I'm a industrial mechanic
by trade so I'm well accustomed to trouble shooting and repairs, but this thing has got me beat..
any input would be helpful. thanks