Full Size Blazer - Sputters off idle

I have a full size 91 with the 350TBI motor. My kid had been running crappy gas in it for a while. I recently changed the fuel filter, installed a Flowmaster 3" system, did the oil change thing , and I run a K&N air filter. The problem is that it stutters off idle. If you step on it, it powers through it just fine. It actually stalled pulling into my garage yesterday. It also bogs down in reverse and when you turn the steering wheel. Any suggestions?