A/c compressor going out (probably due to worn bearings seizing).
At speed, the engine speed i forcing the compressor to turn.
At idle, the idle speed is not "strong enuff" to push against the seized bearings.
Any squeaking noises?
With the a/c on, if you open the bonnet and look at the compressor, you will see that it is struggling to turn, and the belt may be slipping on its pulley and not turning it at all.