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I have a 1991 GMC Sierra.  I put in a rebuilt engine. It has a standard transmission, 305 V8 engine, and is 4X4 1/2 ton. It now has thirty thousand miles on rebuilt engine. Only the engine was replaced as far as I know.  Last week, I was driving, and all of a sudden it started kicking, and bucking, like it would if you were trying to take off in third gear (which, of course I wasn't).  I let the truck sit for a while, while I sat on the side of the road for 1 hour.  It started back up and ran fine.  Having 30,000, we figured we'd do a tune up.  It's important to know that no codes were given and no lights came on. Odd. It was pouring black smoke out of the tail pipe also. Tune up included new fuel filter, plugs, wires, cap, rotor and even the coil.  Truck ran fine for about 1/2 hour and engine kicked out again, leaving me stranded once again.  I noticed some type of liquid coming out of the charcoal canister (the black can like thing by the radiator), thinking it may be unburned gas, thinking it may have been a vacumn problem, we changed the EGR valve (which was stuck closed).  Also noticed the catilic coverter was shot (litterly), so we took that out and replaced that also.  Still, again, it left me stranded, along with litterly blowing up my muffler

It seems like it runs fine when it's cold, but as soon as it gets hot it will start bucking and kicking and pouring out black smoke.  If you let it sit for a while it will run fine after that.  I'm about at my wits end, and sick of being stuck on the side of the road.  Any ideas as to what I should check/replace next?  Running out of ideas.  We did do a cylinder check and all cylinders are running fine.  No codes are being sent out which is making me pull my hair out.  I can't afford to just keep replacing parts.  Not that it didn't need a tune up, cat, and EGR, but to have it all go at once?????  Put a big hole in my pocket!!!  

Made hubby take it to work today.  Now it's on the side of the road again, this time it is not firing at all.  Bout time he had to walk with it.  

Just thought I would let anyone know, incase they run into similar problems, that we found a solution, and it wasn't easy to come by.  Because one minute it acted like it was a spark problem (hence the previous post where it wouldn't even fire) and other times it seemed like a lack of fuel problem.  Well, it turned out being both problems.  The fuel pressure was intermidently low (not all the time) because of a fuel pump on it's way out the door.  Also, there was a crack in the distributor shaft, something this particular motor is famous for.  So, now with a new fuel pump (and of course, sending unit) and a distributor from the bone yard, truck is running great.

I don't know why I didn't think of the fuel pump first.  But I figured that a fuel pump would just go, not be intermidiately working or not.  It's always been my experience that it just goes, usually in the worst place possible.