need some troubleshooting help

here's what i have
94 GT only mods are ;cai,smog delete & off road xhaust.
these have been on the car for a while but this problem has just started. Ok car runs completely fine with no issue's until you turn the hvac control from off to any accessory, whether it be floor,defrost etc.. about 2 minutes later the car feels like it is running on 5 cyl's ,missing,popping back thru the intake,CEL comes on and stays on then it will clear up for a few minutes with a power surge then repeat all over again. After the car sits for a couple of hours it is back to running fine until you switch on the heat. the 3 codes i have (172,173 & 217) which i believe are coming from the way the car is running when it is missing. so my question is what could be the culprit behind the problem that causes it to start missing because i have no clue. Any ideas or advice would be greatly appreciated.

ok but if the O2 was bad wouldn't it act like this all the time not just when you turn on the heat. That's what has me puzzled. Thinking maybe the ALternator is not putting out right when there is a load on the system which i think would explain why the car starts missing because of low amps and not enough juice for the ignition system to work properly. Any thoughts on this or am i going the wrong way?

Thanks for the help. O2 sensor fixed the problem