my 1999 c 250 td overheats only when driving, have had it idling for ages and temp sits on 85 c , changed thermostat, flushed rad , all pipes gets hot so i dont think its the waterpump, no oil in water or smoke or noise, as soon as i take her out and up to 60mph the needle shoots up to 120 and power gets governd to 20 mph! any help please.