I had exactly the same problem, and it was my water pump impeller that had cracked. You can check the water temp at each end of the cooling system using a digital thermometer, the difference on mine was 30 degrees!!!
I think Wrinx got his deduction the wrong way round, the engine is more prone to overheat at low speeds/idle and less so when at spped as the airflow aids the cooling. Mine used to begin to overheat only when in town, but was fine on the motorway.
I had the GTV pump fitted and all the belts done at the same time as they were only 10,000 miles from being done anyway.
S.