Temperature gauge going up and down like a yo-yo
 

I wonder if someone can help,
I have an audi 80 2.3 and the other week I put my heater on and noticed the water dropped in the expansion tank by about a litre so I refilled it with anti freeze and all was well.
Today I warmed the car up and all was well then when I put my heater on my temperature when back down to 0%.
I was thinking maybe an airlock or maybe a new thermostat, I put my heater on full hot and undid the bleed screw on the pipe which goes into the heater matrix but nothing came out.
Also I notice when I undo the expansion tank the level goes up a touch but don't know if this is related.
Any ideas would be much appreciated.

hi vince
new to audis so i could be completely wrong but?
i have a coupe gt 2.2 litre which boiled up on the ramp for mot. i then had the cam belt and water pump done because its just been laid up for 15 years.
the mechanic surgested a new thermostate which considering the long lay up of the car seemed possible. so i purchased one because i don't want it to dump me at the roadside in a cloud of steam in the summer.
i fitted it and when i filled the water up could,nt get any heat from heater and the temperature went up to hot, i could'nt see anywhere to bleed air.
in desperation i looked through the haynes manual which said to loosen the thermo time switch (located where the top hose goes into the engine) and purge air from the system.
i took it out altogether and there was no liquid visible ,so i poured about 2 litres in through the hole and when i ran the car the heater ran warm.
after a couple of purges and filling the header tank when the engine cooled down all seems ok.
i checked the old thermostat and it seems to be working.
maybe these cars suffer badly from air locks?????
mike