RRC aircon drop off at speed

[aRRon]

Aircon gurus,
My 90 RR Classic aircon works great except on a high speed run (eg > 10 minutes at 100km/h) when it seems to stop cycling and no cold air. Also the compressor seems to "load up" and the fan vbelt starts to shriek necessitating switching off.
As soon as slow down to 80km/hr or less all returns to normal.
Compressor is fairly recent new unit, as is receiver/dryer and all new hoses and regas with (shh LPG) 12months ago.

Ideas welcomed.

[bee utey]

It could of course just be a loose fan belt, otherwise the first thing to do is put a set of gauges on the system to see if the pressures are within specs. It may be overcharged and/or the high pressure switch has failed.