I'll throw my .02 worth in and say sounds like you might have some problem
with the distributor vaccuum advance. I had similar problems when I switched
carbs on my '76 TR6 to carbs from a different year (don't ask) and there was too
much vaccuum on the advance. The giveaway for me was misfiring when the temp
gauge hit the halfway mark when idling in traffic. Cold starts were fine and
engine ran perfectly until that point. When I went back to the old carbs with
the correct vaccuum connections the problem disappeared.
Try unplugging the vaccuum hose from the distributor when it's hot and check
It's unlikely that mixture is at fault as it would exhibit other symptoms.
Check the colour of the plugs and if they're tan or light brown then you're ok.
The only other suggestion I can offer is, since it's temp-related, check the
functioning of the temp compensator on the carb body and ensure it moves freely
and is not stuck... long shot but maybe worth checking.