Immoboliser starting to really grind my gears!

Right.... i have a polo gti 6n2..

I've had trouble with the running of it- changed coolant sensor and took it to vw who told me it was either the key . coil or ecu.

They coded the key back to the car and everything is fine!

Now during the cold weather i think my thermostat have died- is constantly on cold unless idiling for hours!

Sometimes when i go to start it - it will start then die- then i try start it again and just keeps turning over. If i lock unlock the car then try it will start but missfire loads :/ - is this the key playing up again or something else. Its quirte embarrassing!!

xx