1988 Chevy Celebrity car misses then cuts out while idle or

I just recently aquired this vehicle and this problem didn't start for a few days. On thanksgiving I reattavched the tailpipe to the muffler and the s belt was loose due to the tensioner pully being loose. While driving apprx 8 miles or so the car missed and seemed like it just shut off for a second. It did it again and I pushed on the gas Nothing it then just shut off. It wasn't a back fire or a stutter it just shut off and wouldn't start back up. I had it towed to my sisters and when I tried to start it it fired right up. On Friday we replaced the tension pully and the belt was good. I thought that maybe it was running on the battery not the alternator and that was the problem. Well I left my sisters to go home and a few miles down the road it did it again. I checkd the codes and got a 15, 42 and 44. I also want to say that sometimes I feel the accelerator continue to accel when I lift my foot and today as I sat in the car with it idle right before it stalled I heard it accelerate then about a minute or so it stalled out. After sitting about 20 minutes it starts right up but after about 15 minutes at idle it stalls and will not start back up.