1993 525 overheating

Hello all.. I have a 1993 525i . Over the summer I noticed the car overheating only when the a/c was turned on standing still at a light or just parked, but if the a/c was off it was fine. Now it is winter time and it is starting to over heat during regular driving. The temp fluctuates at times going from red back to normal temp.. and now the heat blows hot only sometimes.. ?? What could it be.. Waterpump? Heater core? a thermostat somewhere? Any help would be great I don't have lots of $$ to waste fixing this..Any idea what it should cost if it is one of these items?

Thank you
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