rpm and idle issues
 

I have an 02 silverado 4.3. six banger if you will. Just got top end rebuilt. Blew a head gasket.Prior to this truck was having some rpm issues where the rpms would decrease to about 4/500 rpm at idle then bounce back up to normal. changed a couple sensors at the throttle body, then got a couple codes that crank and cam sensors were bad. Changed the crank pos sensor. Now the truck will run fine when still cold but as soon as it warms up it doesnt want to travel over 35 mph and it wants to die. What in the world could be happening?

I would be leaning towards either a coolant temp sensor or even a catalytic converter clogging up.

I really appreciate the feedback. The clogged cat was mentioned by an aquaintance of mine. If I can get it to a shop i just might try that. Is it possible to cut the cat off myself and temp a piece between in the mean time you think???? Once again thank you so much for the feedback.

rpm and idle issues
 

OK so i forgot to mention that recently I now hear a slight rattle coming from under the truck. Never did this before. also i found that the truck keeps throwing out several codes and bad sensor reads. Last time I checked for codes my scanner threw out six different reads , this was about a week ago.

Those codes would be very helpful. I am pretty sure you have pre and post cat O2 sensors. You can pull them and start the truck and compare the pressure coming out of each or even drive it like that and see how it runs. You will definitely get some diagnostic codes popping up, but a quick drive should tell you something.

Oh and that rattle could very well be your converter coming apart - just a guess, but it makes sense.