Heres a video so that you can get a better idea of what is goin on
http://www.youtube.com/watch?v=Qh8ZllPj-iw

 

Under your throttle body is your intake manifold. It is a 2-piece manifold. The upper piece that you see. Take your air cleaner off, grab a flashlight, with the engine OFF open the throttle plates and shine the light in. That's the top or floor of the bottom plate. If it's dry and dirty it's ok. If it's oily and cruddy you have a leak. That bottom plate is thin metal and the gasket fails. That gives you your plenum leak.

 

You said it does that when you stomp on it. Does it do that during normal driving or are you able to get over 45? It is cutting out around 45 and that's kind of what happens when it isn't getting a good speedo signal. It looks like it is going into limp mode because the computer is not getting a signal from a sensor or it is getting a bad signal from a sensor (could be bad sensor or a failing part). Did it do this before you did your tune up?

 

Doesn't do it under normal driving conditions. Only when i try and pour it to it like if im passing someone. Did it before the tune up, thats actually why i did the tune up. Figured it was plugs or wires or sumthin, but it obviously wasn't. What sensor would you think it would be?

 

That would be the cat, my GMC did it only on freeway driving, it was fine during regular driving condition's "45-50" but the second i got around 60 it would die out and throw a check engine. i cut the cat off and it fixed the problem.

 

Hmm, you could try the cat. I was starting to lean towards tranny but, try the cat first.

 

Ok got it worked out. i cut off the cat and gutted it. Welded it back on and it fixed it. Thanks for the help guys.