Air-conditioning in GU 4.2D

Hi all, we appear to have a problem with the aircon in the Patrol, It works great normally but to get home we have a large hill to climb. Today coming up the hill a woman in a mirage (car) chopped in front of us and sat on half the allowed speed limit, now we know that to get up our hill in a diesel we have to get a run up and keep it up, but because of this 'lady' coming out in front of us we had to drop back a gear hence our problem, the aircon cut out and stayed out until we were nearly home (about 4 mins) then it started to cut back in for a few seconds and then cut out again, by this time we were home.
Now we also know that for power sake (and we usually do) one should turn off the aircon before heading up the hill but it is a bit hot over here today and we had a patient in the form of our dog in the car who we had just picked up from the vet after having chemo so it was important to keep the cool going.
This is not a one off, it has happened before. Any ideas or is there some sort of cut off switch if the engine is under load which we were not.

Thanks in advance

Cheers

Deanna