Car just won't do anything now

I own a 98 Pontiac Sunfire GT, 2.4L, 205,000km. I just got the repair pipe fixed from the catalyc convetor. Since a few days after driving the car would jerk and die while driving. I boosted it and put a water remover in cause thats what the repair place told me to do. I also put premium gas in and it stopped the jerking and drove well. Then when the gas was low I put regular gas back in and it started doing it again. It was boosted and wouldn't start then did and died now it won't boost, start and sounds like it is going to but makes that putt putt noise and sounds like it is backfiring a bit. Any help would be wonderful.