1998 Chevy Lumina Engine misfiring on heavy acceleration

Engine problem
1998 Chevy Lumina 6 cyl Automatic 130000 miles

I got this car from my grandparents last year. Car was running fine, until the past week. I was driving on the highway and felt the engine misfire. I have tried using fuel injector cleaner, which helped initially, but now the engine will misfire on heavy acceleration. If I accelerate slowly there is no misfire, but the service engine light remains on. I am very tight on money right now, but if I can get enough mileage for my money to fix the car, I will do it. Over the summer I had the car fixed for (if I remember correctly) a blown gasket. It basically sounded like the engine was running like a lawn mower (it was running "clunky" and not the smooth typicall hum. I think the mechanic replaced the spark plugs but I can't quite remember, but up until last week it had run fine. Any advice would be much apreciated and maybe an estimate of cost. Thanks