2002 Pontiac Sunfire Hesitates and bucks over 50mph

I'm having a semi intermittent problem where the car bucks and the engine severely hesitates when operating over 50 mph after running for a while. I used a bottle of Gumout fuel system cleaner 2 weeks ago and the problem seemed to go away. I commute 80 mile a day and it performed fine for over a week but yesterday the problem returned. I drove it to work this morning without any problem but it started again on the trip home after driving 35 of the 40 miles. I worked as a mechanic when I was younger, but had to give it up due to a back injury. A mechanic friend of my son said it might be the "coil tower". I've already purchased a fuel filter that I plan on replacing this weekend along with the spark plugs. This is our only vehicle and I don't have the money to take it to a mechanic. Any help would be greatly appreciated. Thanks in advance.