I cant do that --->

after driving on the interstate for 30 or so minutes.
The longer I drive on the interstate lately, the worse my clutch seem to operate- to the point that it won't disengage. Would this be a symptom of air in the lines or a worn clutch? slave cylinder? I tend to think it must be heat related.
If the truck is stopped, the problem will correct itself quickly (within ten minutes or so). Aaround town it's fine.
It's a 90 GMC with a 350 and 5-speed. I would appreciate any help anyone can give me.
Jack