All mended - in brief to help anyone else who may get this fault.

Engine stumble at idle when aircon on caused by radiator fan not coming on intermittently generating high pressure in air con system and shutting down a/c compressor, which made engine stubble.

Replacement radiator fan from Sam @ Mopar $550 including postage to Qld, fitting $88 Re gas $88. Re gas not necessarily required by I had it done, as it was 4 and a bit years old.

Incidentally I also had a faulty low pressure switch that was replaced for approx $90.