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If you still have your old carb around why not give it a try?
Have you swapped the condenser?
Have you jumped the ballast resistor when the problem occurs?
Inspected the distributor cap for a crack that opens up with heat?
Squirted oil everywhere there could possibly be a vacuum leak?
Are you using a Sears security battery?
OK, I' really reaching now!!! What about your timing chain? The "bucking" got me to thinking about what could cause that to happen. A loose chain will cause eratic timing at idle, but under load it will be fine as there is tension on the pull side of the gear. OK, that's it for me. Good luck.
I think it's time to kill this thread... Lots of good suggestions and ideas, but we are all running out of steam. I'll play around with all the stuff I haven't tried yet and let everyone know what it was when I find it. Thanks to all.
BTW, the timing chain is new.