Auto Air Conditioner

I have a 2002 Seville SLS with 67000 miles on it. I purchased the car in 2002. Two years ago I developed an AC problem. When driving around town the AC works great all the time. On the interstate I set my Cruise control at 70 mph and Every 10 to 15 minutes the AC starts blowing warm air. This last for about 15 seconds and it gets cold again. I live in Florida so the road is pretty flat but it keeps doing this as long as I stay at 70 mph. I can slow down to 50 mph and it cools all the time.

I took it to the dealer 2 years ago and without even running a test the dealer said I needed a new compressor. At that time the car had 46000 milrs on it. I have since been to 4 AC garages and they all told me theres nothing wrong it’s supposed to do that. I think they just don’t know what the problem is. It didn’t do that the 1st 5 years I drove the car and it hasn’t got any worse in the last 2 years. HELP!