LUVTruck.com

Two things as about all the common stuff has been covered already. The air cond compressor cycling on and off, (I know you said the air was off but disconnect the wire to the compressor clutch just to be sure).
An internal leak in the carb causing fuel to drip into either the primary or secondary throat at a regular rate. My two guesses.

Egg has a point...
This does sound like a fuel system problem...
Maybe there's a small crack in the carb somewhere and when it warms up, it opens up enough to release the puddle of fuel behind it, and after a bit, the fuel cools it back off and closes it up.
But I still can't see this being the problem as the intake side doesn't heat up that much... that is unless you don't have the cross flow heads? (yeah, I know they were phased out waayyy before '79, but who knows what people may have done to your truck over its lifetime...

Good luck! Just noticed the pic of your truck, and it looks great... red interior w/ black... wow...
Kinda makes me wish I wasn't the 4th owner and the only one that hasn't ever hauled firewood with mine...
-ret

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1980 4X4 LUV daily driver, as stock as can be.

One week since I got the Weber on and my truck up and running. Do not have any of the problems I had like yours anymore. No surges, no stalling, no misses, no feathering at a stop. I agree with Egg, my thought is my old stock carb had some sort of heat expansion type leak in it, which may be your problem.