Overheating Passat TDI 130bhp 02 plate 105k miles on clock?

My passat runs oK until car is doing approx 80, at which temp goes red on gauge and dashboard warning to stop flashes up. If howvere I moderate the speed the temp drops, would this indicate a dodgy water pump? I have just had it check out at garage, no leaks and they thought it fine. Any ideas what to do/ check next please?