Joe,
I say again, have you run an air conditioning diagnostic system, as per manual 4 - 7 - 4. ?
The sensors can be checked aa per the manual instructions, using an ohmmeter and thermometer and is not difficult. My money remains on an intermittent sensor or circuit, first the in vehicle/cabin, then the external ambient and least likely the sun-load photo diode.
I am now wondering if your UK cars are different from my JDM. as is covered by manual Publication No. G301RE(4) Feb. 1992, details on back cover.