overheating tdi, but a strange one

[martin_jg]

basically my car started creaping past the 90 mark dangerously towards the 120ish mark after a relatively short drive.  since then it keeps doing it.
now the strange part (to me anyway)
as i stop or pull over or leave it idling in traffic, the temperature goes down to 90ish, but as driving thats when it increases?
i have changed the thermostat to no avail.
im getting worried as im thinking headgasket, but the top hose to the rad is warm, and bottom cold, the fans cut in when it got very high temp, but not as it was just creeping over 90 as i could see?
my fans work on hot and cold, no smell of oil in water, and no strange mixture of oil in water or water in oil?
my next point is to take it to a garage and get it on the diagnostics, but i know how much this costs, and my dad is a very competent with work on the car, so i was just wondering if anyone has any ideas?
please help
thanks

[barrym381]

had the same problem on my 1.8t an it was the waterpump

[Wayne]

Airlock ??

[javalin]

yeah - airlock would be my guess too - when the engine is up to temp, both top and bottom hoses should be hot and "hard" - i.e. full of water under pressure.

Start it from cold with the expansion cap off, let it warm up and massage the hoses until all hot.

Turn off and replace cap.  Job done.

James