Pontiac: 1999 Grand Prix, hesitation, missing, grand prix gt, gt 3

Question
1999 Grand Prix GT, 3.8 V6 (Standard Engine) 89000 miles.
I have had a problem with the engine hesitating and missing. It does not do it all the time. For example when you are running 55 mph on the highway the car will miss and hesitate when climbing hills. You can press the gas and accelerate out of it. The problem is getting worse. The check engine light never comes on. I have taken it to a local shop where the problem was thought to be the plug wires. They were replaced along with the plugs, coil packs and fuel filter. I am beginning to suspect the fuel system. Can you give me any advice on where to go from here.  

Answer
The most common problem is partially clogged fuel injectors.
You should take it to the dealer and have them cleaned. It should be done at least once a year. They need to be cleaned under pressure so using any of the fuel system additives that are out on the market wont do any good and are a waste of money. You might want to have it diagnosed correctly at the dealer before you spend the money to have the injectors cleaned.