2000 Nissan Altima Many repairs, car is worse

In Jan of 08, the service engine light came on, went off, came back on, took it in to mechanic. They did the following on the car: knock sensor, fuel filter, 1 fuel injector, 4 platinum spark plugs, 1 ignition wire set, dist cap, rotor, tune up. 2 days later, light came back on. They turned it off. Came back on. They turned it off again and also adjusted the idle. By the 3rd time they said they could try something else but that would have been another $450 on top of an already $954 bill and there were no guarantees that was the problem either. Now, the car runs fine except that at stop signs/lights, it would "buck". Took it in yesterday, was told 2 of the 4 O valves were letting in air so replaced them. Picked car up and it didn't want to start for mechanic but finally did. Drove it home and noticed when wanting to accelerate, there was a huge hesitation, would not speed up, finally would and drove fine at 55. Went to pick up kid an hour later and had a horrible time starting it again and again the gas pedal did not want to respond to my foot. Took it back to the mechanic. He worked on it for 1 1/2 hours. He replaced 2 spark plugs and when he pushed down on the fuel injector, the engine sounded normal. He suspects it may again be a fuel injector (the 4th one) because when he unplugged the spark plug tubbings, the engine didn't even respond to that one being unplugged. The engine did seem to die down when the others were unplugged. Drove it home but again, now, to start it, I must press the gas pedal to the floor to get it started. It is still very hesitant to pick up speed. The delay is very noticable and also dangerous when you NEED to go and the car does not move. Another mechanic, not having checked the car physically yet, thinks it may just be spark plugs, that carbon built up and won't allow the plug to ignite. ( I hope I remembered that correctly). In any case, my car ran better before getting the O rings fixed. At least then it started and went when I pushed on the gas. Can someone give me sound advice? I just want my car to run like it used to before January. What would now cause the car to not start and not go when the accelerator is pushed?