88 tbird sputters and dies @ low speed

My 88 tbird was having problems idleing. It would start then would die instantly. It had never had a tune up that i had known about so i decided to try new plugs and fuel injector cleaner. While i was changing the plugs i found a fouled plug. I replaced them all, now my tbird still dies like it had before but idles rougher. But on the other hand, it has alot more power and drives very smooth when i hammer down on the pedal. Do you think the gap on the plugs are wrong??? I have no idea. Im open to all suggestions. Thanks a bunch