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?