My 96 m3 sometimes overheat after i have been driving for about 30 minutes. It seems to happen if it idling or if i am just cruising in 4th. As soon as i start to accelerate or go down a hill it will go back to normal. When it overheats it goes to about 3/4 line on the gauge. It never goes over this. I recently took it to the dealership but they said they couldn't duplicate the problem. They said the tested the radiator cap but it checked out ok. Is this bad for my engine or is ok since it never gets in too the red zone.