It sounds like fuel starvation. Look down in the tank with a flashlight and check to see if the fuel outlet is partially blocked.
Then check the fuel flow to the carb. Remove the fuel line and check for a strong flowing stream about 1/4" dia.
If that is good, clean the carburetor really well using Berrymans B-12 and clean all the little passages with a stiff wire.
Check the float setting to make sure it is allowing enough fuel to enter the bowl.
I doubt that your float is sunk due to the symptoms you describe.
If that doesen't help, a good, complete tune-up seems to be in order (points, condenser, rotor, cap, wires and plugs).
If the problem isn't fixed then, I would suspect the coil. Sometimes they will act really wierd after they warm up.
P.S. One more thing. Be sure you have fresh gas.