I have a 70 TA with a RA3 400 and A/C. The car runs at 180 in traffic with the A/C on but on the highway the temp rises slowly the faster I go. At 40 mph it is fine but as the speed increases so does the temp. I re-cored the 4 core radiator and that helped.  

but didnt solve the problem. What should I check next?  

Check to see if the bottom radiator hose has a spring in it if not the hose could be colapsing at highway speeds.  

Yes it has the spring.  

That would to believe that the fluid in the core is not properly cooling down. Once u get to highway speed ur now pulling the rad fluid into the block and it never proprly cooled down. sounds like air flow or fan probs. I know u said u had the rad cleaned or sum thin 2 that affect but it sounds like it needs 2 b re addressed.Perhaps the clutch fan is gone or u have some rd kill caught under ur car   LOL.  

Sittin here drinkin some swill and man it sounds like a clutch fan prob. Sounds like the RPM in the motor is not matching to the fan, clutch is slipping 2 mutch @ higher speed, That would sound like the driveway mechanic approach but then again u could take the rad out and put a alum rad in w/elctric wtr pump by one of a couple of Co. and Electric fan w/ temp stat. Wow about a a grand later.  

This forum would not let me back to reply, sorry I took so long. The problem turned out to be the fan clutch. I found one that works at pep boys auto parts for 50 bucks and the car runs at 170 degrees with the a/c on. Thanks for the help.  

Hi There, I run a 1973 Super Duty 455 in my '80 trans am indy pace car and have been experiencing the same issues with over heating at speed. Is there a way of checking to see if it could be the clutch on the fan or do i need to just bite the bullet and treat myself to a new one? Bearing in mind that i am in the UK and these parts can be pretty costly - especially if not needed. My issues also go a step further, that the engine can overheat to the extent that the expansion tank fills and the coolant overflows - which only makes the problem worse - do you think this is just caused by overheating the engine or do I have to dig deeper to find any other problems? And I'm sorry to go on but i also note that on occasions the top water hose(connecting with the thermostat) will colapse on itself? Any help gratefully recieved.  Andy  

Just try a new fan clutch. I also re-cored my radiator and replaced the thermostat. I had her out all day in 90 degree weather with the A/C on and stayed at or below 180 even at 90 mph. In traffic she runs cool also .