80 Series A/C question

I have a 1994 80 Series Landcruiser with climate control. The A/C seems to be playing up - running hot and cold (literally). Doesn't seem to be the thermostat, as the fans adjust as you change the temp setting.

Am thinking it is compressor or belts. The engine seems to 'clunk' when A/C is switched on. Could it also mean the the A/C needs to be re-gassed? Advice would be welcome, thank in advance!