i have a 1991 grand am 3.5, it has ran great for years. now it is running when we let it set for awhile. then when i start it , runs for awhile then it seems to struggle getting fuel to the tbi. i changed the gas filter, spark plugs , still not changed. it runs but not long. i was wondering if it would be best to put in a shop and have it tested for electrical sensors. i dont think it is the fuel pump. i took the injector out to flush out the gas line, to make sure it was not blocked. and to see if gas was coming to it. it pumped hard and fast. that told me it was not the pump. tell me where to start looking for a problem in the sensors or what you think this might be. i know it is something simple, it usaully is. two years ago i replaced the fuel injector, i broke the first one. i was wondering do these go bad, i checked the screen in it, but it looked relly good. it seems to spray like it shoud, but still dont understand if it is in the fuel tank. i thought it could be stuff in the bottom of the tank stopping the fuel from traveling after driving and shaking up the tank. i filled the tank up half way, and put a bottle of fuel line cleaner, to see if bringing the level up would help, it did not. what steps should i take to find out if i need to take the fuel pump out, clean or replace it? i need help. what would you do?

Optional Information:
1991 General Motors grand am 3.5

Already Tried:
fuel filter, plugs, injection cleaner,