We are having similar problems with a V5 Golf, came in with a clutch problem and an over heating issue.
We couldn't get it to overheat, had it running for ages, drove it for 15 miles - no over heating. explained the lack of overheating to the customer. Customer collected the car got up to speed on a dual carriageway and the car started to heat. Under 50 mph and no probs, go over and it starts to overheat. We checked the waterpump but that was fine. Not sure what to do next? Any thoughts.