2000 Holden Commodore persistant overheating

I am not a mechanic, just a housewife frustrated with a car that keeps beeping at her, and looking for help. My car continues to indicate that it is heating up. The temperature gauge just goes up and down, not erratically, and seems to be in tune with something kicking in and out in the engine. A fan perhaps? We have taken it to a mechanic over and over again and have spent a fortune fixing things that aren't the problem. Most recently the temp reader itself.I have found that when it is on high and stays there for more than 5 seconds, turning on the air con actually brings it back down. My thoughts are that this sucks fresh air into the engine forcing out the overheated air. It is definitely heating up because if the ac is on prior to the overheat, once it hits high the air coming thru the vents is warm. My husband thinks it may be electrical, ie a message to switch on a fan to cool the engine is not always being received, so our next trip is to the auto electrician. I am not entirely convinced. Any ideas?