1983 Jaguar XJS Involuntary starting and stopping of aircon

Hi - my aircon decides when it starts and stops - the switch needs to be on for this to happen but then there is no guaging when the system will start working and when it decides to stop. When it is working - it is fine - the only question is why does it have a mind of its own?
I suspect that the probelm may lie in the swtich - but then that is only a guess - the switch must be working otherwise it wouldn't come on when it does.

Any help will be appreciated.