[jhv48]

Sounds like vapor lock. Boiling fuel in the float bowls. Does the carb have a spacer between it and the intake manifold?
If you have a switch to turn on the radiator fan, (the one on the back side of the radiator) try leaving the fan on while driving in stop and go traffic and see if that helps.
Try the simple things first. Since it runs great until heat builds up, that should be the culprit. And with less than 1,000 miles in 4 years, my guess is the previous owner never drove it long enough for the problem to surface.