Over the last couple of years my air-conditioning has intermittently cut out until this summer when it only worked for 5 minutes from starting the car from cold. I had it re-gassed but it turned out that the condenser fan had failed. Opened it up and the motor was shot. So I replaced both the engine cooling fan and condenser fan at the same time and the air conditioning worked brilliantly. Except it only works brilliantly up to 60mph. 1-2 mins of motorway driving and it cuts out until I slow down and then it re-starts. What is causing this?