Need Help Bad! Mechanic cant find problem!

[NotchJohnson]

Yeah I was going to go carbed when i first decided to build a motor because my buddy was running a carbed 306 and it was making awesome power, he told me it was cheap so i figured thats what i would do. Since then ive gone off to school and have decided to keep the car efi, since I take the car 180 miles to school. I have added a few mods to the stock motor such as a cold air kit, exhaust and all the small stuff. I have a shortblock for a 331, I just have to wait until summer so i can get a good job and finish it. Anyway I did drive the car earlier and it runs fine at cruising speed now. what the hell! last time i drove it it ran ok for a few mins then got crappy. I just drove the car 25 miles and the damn thing only acted up at idle. This is really ticking me off. I am going to hook up the o2 sensors tomorrow first thing. Does it make sense that the car ran ok for almost 2 months without them being hooked up, but now suddenly went screwy? Thanks for all the suggestions and help because im pretty lost. -John