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Change, plugs, wires, front O2 sensors, upper/lower intake gasket.
Problem with a 1999 GMC K3500, 7.4l v8. Didn't pass CA smog due to high hc at idle. (277). The mechanic did some tests and said the intake gaskets were sucking air and replaced both upper and lower. Before this, engine had no performance problems, other than a tiny hiccup type miss at idle. He tells me to drive it a day before I retest for smog. On the way home, I notice the temp gauge is not working, so I look under the hood and see the head temp sensor? is unplugged--plug it in, and temp gauge is back. Next day before retest, I take it on the interstate and when I hit 70, there is a major power loss and I can barely make it to the next exit. Take it down below 50mph, and it works fine again. Drive in town, it works fine...over 50 is like sticking it in neutral and watching the speedometer fall back, while the rpms are higher than normal. He tested it and said half the engine was shutting down at 2500 rpms and at idle it says its going 84 mph. He thinks!
it may be pcm, but I think it sounds hinky because nothing was wrong with it before he did the gaskets. Did he forget to plug something else in? It had new plugs, wires, oil, front 2 O2 sensors put in and was driven on the interstate no problem the week before. Anyway we checked smog just to see if the intake gaskets helped and idle hc was even higher. At this point, I just want my truck back the way it was, but it'd be nice to have that and smog too.

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Don't see how replacing intake gaskets can cause this power loss.
If mehanic left something else unplugged, it would set a check engine liight and codes.
Suggest getting fuel presure checked. May have a plugged filter, or fuel pump going out.
It is possible that while doing the intake that some debre got down in the cylinders and plugged up the catalytic converter.
If the RPM are very high, and not accelerating, then this would be transmission problem.

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Make sure comp. is reset , by disconnecting battery for 30 sec. or so, but also sounds like cat. converter..

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