1.8t SLX, 2000, 49K km.

The other day arriving to work the temperature gauge went all the way up to the red line and the dash light and sound alarm turned on. I parked the car, later that evening I returned home under heavy traffic with no problems.

Today I was returning home under heavy traffic with no problem, but when the traffic begun to clear and I accelerated the temperature began to rise almost to the red line.

Any thoughts?

waterpump failure

Could also be coolant temp sensor failing, which is a very common fault.

Check to see if you have flow from the small headertank return pipe, if there's no flow your water pump has failed.

Seen a couple of threads on here and they all seem to point at the impeller on the water pump coming away from the shaft it's on - low revs there is enough grip for the water pump to work correctly but at higher revs it can't hold and the water stops flowing 

What mikec said, I've had it on 3 VAG cars. Plastic impellors utter rubbish.

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I strongly recommend HEPU water pumps with metal impeller,whey are bullet proof.