Weird "bucking" issue

[kaysboat]

I'm starting to have an intermittent (although more frequent) "bucking" issue with my 1980 C10. (250 straight six, three on the tree). Initially when I start driving it drives very smooth and normal. Sometimes after 5-10 minutes I start getting a relatively major shaking/shimmy through what feels like the drive line. (When I engage the clutch and rev the engine engine is smooth and sounds normal, although shaking continues).

The shaking inceases and decreases with associated speed changes. Usually I pull over for a minute (leaving the truck running), then the problem almost always fixes itself. Sometimes it is fine for the remainder of the drive, sometimes the issue starts again in a few minutes.

I just had the clutch replaced in the fall, and had the flywheel machined at that time. The only issue I can see currently with the truck is a small hole in the exhaust pipe (about the size of a dime) just in front of the muffler (have to get that fixed up soon). Although I doubt that would cause such an issue, but I'm not sure. All wheel weights are still on the wheels (although I don't see how a wheel weight flying off would cause an intermittent issue).

Any thoughts?

['76 Big Block]

Was the throw-out bearing replaced when the clutch was replaced?
How much free play do you have before the clutch engages?
The shaking with the clutch engaged indicates clutch/bearing issues (to me)----good luck!

[kaysboat]

The throw out bearing was replaced
I meant to say "disengage the clutch" and rev the engine. Engine runs smooth even with the clutch disengaged, but shaking persists.

[geezer#99]

Sounds like your heat riser is stuck or not working and you have a cold intake plenum.
How long do you let it warm up before you drive away?

[kaysboat]

I usually let it warm up for 2-5 minutes. Basically once it no longer would stall without giving it a bit of gas.

[geezer#99]

Is your motor all stock?
Is it an integral head motor?
After you stop and sit a bit and it fixes itself, does it run much better after that with no bucking sensation?