About a month ago my car started to shudder when I pressed on the gas peddel. In first and second when I press hard on the gas pedel it won't shudder but when I shift into 3rd and the rpms move down to 3000-3500 and I press on the gas hard the car starts to shake! And when it shakes the rmps stay the same even though my foot is all the way on the gas....and then when the car stops "shaking" it just speeds up like it should. It'll also do this in 4th and 5th. Its only when I put the gas down more then half way (I mean like not slowly putting it to halfway i mean like when your trying to accelerate hard and you just put it halfway right away) in third, fourth and fifth gear. I have a 94 base 3000GT. I hope I explained it good enough. So after this started to happen I took it to valveline and got the fuel injectors cleaned...this didn't help....so I think it might be the wires or spark plugs???? Sorry for this being so long but I would really appreiate any input....its so damn annoying when it does this. Also I was thinking of putting on a Pilot motorsports muffler with custom piping...would this sound good?? Thanks for the help!!!