I have a 1997 catillac STS with a northstar engine, with a problem of overheating. The first sign of overheating is when your up to highway speed(60-75) with A/C and it happens(sometimes, but sooner than later)
          
               Things that has been done are:
               1.) Pressure checked for leaks and headgasket.(OK)
                 2.) Checked operation of cooling fans.(OK)
                 3.) Radiator backflushed and new antifreeze.(DONE)
                 4.) Replaced thermostate.(DONE)
                 5.) The engine runs fine with no noises or rouph running.
What else can I do?