Matt,
Possible the butterflys ends or the throttle linkage where it goes through to open them is getting bound up when it gets hot. Another thing is if your throttle return spring is losing its tension when it gets hot. When I was racing I ran two return springs just for that reason. You can get them isn all different tensions so it was easy to find two that was the same as one stiffer one.
A better place for this post would be over in the All Cobra Talk Forum as thhose guys will definitely know what to tell you.
Ron
