Does it run better once it cools back off?
Is the fuel tank vented properly?
Is the intake / exhaust manifold bolts tight and not leaking?
Is the vacuum advance working?
Is the fuel line right up next to thermostat housing?
Tank not venting could be creating vacuum in tank, starving the engine,
Manifolds cold be letting in air where it's not supposed to be once they heat up,
Vacuum advance can play havoc on the mixture,
Fuel lines are close to the high heat of the cooling system on these engine creating vapor lock.
