M151A2 clutch question

About 800 miles back I put a new clutch into my Mutt. I used the puck type clutch disk.
I also replaced the bearing and bolts, all the stuff that comes with the clutch kit.

Adjusted to the specs in the manual.

The problem is that the vehicle runs fine in all gears for about the first 10-20 miles. After that, it shifts just fine in 1st and 2nd, but you feel slipping in 3rd and 4th.

I have checked the adjustment to be sure it has not slipped, and played with adjusting it up and down to see if that made a difference. It doesn't. Now I did not resurface the flywheel, which some old timer here at work told me I should have done.

Does anybody have any thoughts as to what the problem might be? I do not see how I could have cooked the clutch so quickly, especially with nothing but surface street and parade use accounting for the miles I have clocked since the repair. Oh, Iand I also replaced the rear seal about 500 miles ago. Oil is not dripping out of that area any longer, which rules out oil slinging onto the flywheel.

Thoughts have ranged from a throwout bearing sticking when it gets warmed up, to I just flat out do not know how to change a clutch.

Positive feedback would be most welcome!

not cutting the fly wheel will cause chatter when you let off the clutch, maybe this is what your experiencing? i've changed a few clutches with some real grooved out fly wheels and never experienced any problems. did you check the fork height? did you adjust? did you replace the pressure plate? i know the new pressure plates don't come preset to the m151.scratch the sticking through out bearing theory, if it was sticking your pedal would be sticking as well . take a measurement off your old pressure plate start there and move on down the line mite just be as simple as some fine tuning. good luck

Is there any oil leaking out of the bell vent?? Sounds like maybe a rear main leak. The pucks hold better and maybe after a few minutes running fresh oil on the flywheel is causing the slipping. Something is causing it to let go after a few minutes and a weak pressure plate or bad clutch would not do that.

You either have the wrong disk(why would you even use a puck disk?) or a bad pressure plate. Even though you did not surface the flywheel, a new disk should not slip with a new pressure plate. The puck type Disk usually has bronze facings which tend to grab better when hot.There should be freeplay(at least 1/8-1/4" at the release bearing.
Remember, as rpm's increase, centrifigal force is affecting the position of the release fingers on the pressure plate.
Just my as a very seasoned drivetrain specialist.