My S3 ( Year 2000 210BHP) has started to overheat!

It only starts to overheat when the car is being driven, on standstill its fine!

When it starts to overheat if I let go of the accelerator the temperature starts cooling down.

I have had the Electric Waterpump situated on the Radiator changed and this has not sorted the problem.

Could it be the main Waterpump???

All help would be greatly appreciated.

Thanks