Thermostat / Overheating problems

Hi Folks, Looking for a bit of help please

So I've recently changed the head gasket and a few other bits an  pieces on my 1982 mk1 GLS cabrio, and a problem it had previously still persists.

The engine will overheat if left to idle. It appears the thermostat isn't opening when it should (or at all for that matter), the temp gauge gets past the middle, and still the bottom hose is cold. I've gone through 3 thermostats, and I've tested all three in hot water, and they actually all seem to work to varying degrees.

I have a top fill rad, which would seem to be the original one. Ive drained and refilled the system numerous times, I've run it with the cap off, topped it up.. all sorts. It still seems the thermostat isn't getting hot enough to open up..

Any ideas would be greatly appreciated.
Oh, and the water pump is definitely working. 

http://www.vwgolfmk2.co.uk/forum/viewtopic.php?f=37&t=14913

you might just have to wait a bit longer, as long its open before gauge gets to about 3/4 all is well. it may just be gauge is over-readign slightly

Is it actually boiling up and loosing water?

If you have a spare lying around cut the centre out, and refit so it's not got a stat so to speak. If problem happens again then you will have to dig deeper. Let me know and I'll come up with something else.

Thanks for the replies!

I'm a bit reluctant letting it run past 3/4, but i might just do it and see. Just doesn't feel right.

It hasn't boiled up no.

Prior to redoing the engine, I was running it without a thermo at all, which worked ok, just took a bit longer.. I seem to remember it overheating in traffic before that.

 I'll give your suggestions a go and get back to you!  

As long as the fan kicks in, sounds like its working OK to me.

is it a top fill or header tank setup? if top fill they can be a git for airlocks to help pop the upper hose off the rad and pour coolant directly into the block till it starts to back up the hose then refit and fill the rad as normal.

if its a header tank setup no such issues just top the tank up and keep it topped up thru the bleeding procedure in my guide above

Make sure the heater is set to hot and as rubjonny says they are a pain to bleed the air out so top radiator hose off and pour in coolant.
When you start the engine check the hoses are all getting hot and the heater blows hot air in the car, the bottom radiator hose should be the last hose which gets hot.