Good troubleshooting.
Sure has the symptoms of vapor lock. Do your fuel lines run in the same manner as they did when stock? Not touching any heat source?
Could be a sticking float. You running a Rochester B carb?
When it acts up again pull the AC and check for the accelerator pump squirting fuel in the venture. IF it doesn't the floats may be sticking closed, if it does check all bolts on carb and manifold for tightness, could be a vacuum leak when everything heats up and expands.
This is a head scratcher LOL
