rover 75 2.0 v6 auto 2001 x ?

recently bought a rover 75,with a over heating problem,previous owner has had it pressure tested and found no head gasket leak,checked inside oil filler cap no "mayyonnaise" dipstick shows clean oil,and header bottle has clean water ,no oily sediment,it seems upon driving the car,under acceleration,whilst a cold airflow is being driven against temperature stay normal,it seems the temperature gauge rises to red light coming on and computer advising engine over heat,when moving again the temperature goes back to normal.
any one got any ideas to solve the problem, all ideas will be greatly appreciated