99 GT Bucks and Hesitates at 2000 rpm with AC on.
 

Starts bucking and hesitating at 1800 to 2200 rpm and has erratic idol only when
air condition is on. Turn off the air and she runs smooth as silk. Went to dealer, hooked up their onboard computer and said number 2 and number 4 plug coils were breaking down. So, I first replaced all spark plugs since they had 80,000 miles on them. this changed nothing. Then I purchased two used plug coils and replaced them. This too changed nothing. Help !!!! Do I need to replace the whole set of plug coils??? I also cleaned the throttle body and idle control valve.
What do I try next. Thanks for any help.... Todd C.

RE: 99 GT Bucks and Hesitates at 2000 rpm with AC on.
 

Test your charging system, it could be causing it to act funny due to the extra amperage being drawn by the a/c compressor, electric fan, and also the blower motor. Also, make sure your battery terminal connections are clean and tight.