Loss of vacuum in the a/c controls which revert to the defrost setting. Check all vacuum lines under the hood for leaks, the vacuum storage tank and possibly for a bad check valve in the vacuum line from the storage tank to inside the car. When driving at high speeds, the engine is operating under low manifold vacuum and the a/c system relies on a vacuum storage tank to provide what it requires....a check valve in the vacuum line also keeps the system from bouncing back and forth during periods of low engine vacuum.
