Help with my vehicle?

i have a 2000 chevy tracker with 250k miles on body and 80-90k on engine i bought it from my boss and it ran fine than it started to putter and die on me after driving for a while i change the fuel pump relay sparkplugs airfilter oil brought it to goodyear and they did combustion buildup in intake and throttle body. runs fine for a while than it starts puttering almost like a back fire (FART) than about 20 minets later it will start again. it seems like if left at idle or low speed it has trouble but after while will start again. doesnt overheat or anything idk maybe fuel pump oh i also put in a fuel replacer like chemical from napa to remove any buildup in tank. please help