Since the air temp seems to be affecting it, here's a few things to look at.
1) Thermostat operating properly
2) Check the Choke pull off opening the choke at operating temperature.
3) Check the Timing and vaccuum advance for proper operation.
4) I don't know what kind of air filter you are using, but if it's stock, Check the vaccuum operated diverter valve on the snorkle. It should be drawing air in from the heat riser during warm up, then from the intake horn at operating temp.
5) Check the carb to intake gasket for any vaccuum leaks.
6) Check the power valve on the carb. It may not be set rich enough. Cold air is denser than warm air, so it needs more fuel to keep the proper fuel/air balance. It might have been a little lean to begin with.