intermittant overheat

[ss2115]

Struggling with an intermittent overheating problem on a 1997 MkIII Golf - 2litre AGG motor/auto trans with air con.
Car been reliable for two years but as of about three months ago occasionally would have a rising temp guage but only if the air con was on. And even then not every time - just occasionally.

Once the temp starts to rise though, it keeps going unless the car is stopped and let rest a while.
Turning air con off doesn't help much on its own, but then turning heater on with fan at full blast does help stabilise it.
Water doesn't drop - on max in reseviour all the time while hot and about 1/3rd when cool.
Fans cycle on/off correctly. Belt between the driven and slave fan is okay and doesn't seem to be slipping.

Have purchased a new thermostat to install this weekend. I'm betting it might be getting stuck. Pretty sure it closes okay because engine warms up quickly/normally.
Intend to back flush the radiator as well and make sure it flows a decent amount of water.

But thought I'd ask while I'm working on it this weekend if anyone knows of anything else to cause this intermittent problem?
Is there a special sequence for bleeding air out of the system?

[SimonG]

The thermostat may help with your situation and flushing the radiator is also a good idea while there is no coolant in it.  I was going to suggest a faultly aircon temp sensor on the coolant system but then you said it happened occasionally without the aircon on.

The only other thing is whether the coolant pump is any good.  When was it last changed?

To bleed the system of air, I usually pour a load of coolant it and let it settle. Then, I squeeze and pump a number of hoses, mainly the top radiator to rid it of as much air as I can and will start it up and let it run.  Some people pour coolant in and run the engine and topping it up as you go on keeping an eye on the level in the reservoir.  Then replace the cap afterwards.  Pretty straightforward really.

[gasketface]

After draining the system completely on a mk3 i always remove the small pipe going to the expansion tank before refilling.Then i start the engine up and hold the pipe straight up as high as i can whilst slowly refilling.Once the bubbles stop coming out of the small pipe i re-attach it and refit filler cap.The i run it up,let it go through fan phase a couple of times then turn it off to let it cool enough to remove filler cap again.Then i top level up,run it without cap for a minute or so and then refit filler cap.

[ss2115]

Thanks Guys.
Will look at this job this long weekend.