[fredjacksonsan]

I had the exact same problem on a car that I had...we rebuilt the carburetor (incorrectly, it turns out) and it would run great until it warmed up, then have absolutely no power.

IMO a new carb will take care of it; but before you go to that expense, check all the vacuum hoses in the engine compartment, on some carbureted vehicles lack of vacuum can cause issues.