Car Running Cold !

Any Help ! My 3 series (2000 model) seems to be running cold when i drive it at 30mph or above, the temperature guage goes to mid point but the minute i drive at a constant speed it drops to just above the blue colour and the internal heating system becomes poor, the car runs fine but it seems to affect the level of heat from the internal heater if switched on......any help as to what going on ? Thanks

Seems like you have a stuck fan or a stuck thermostat. Stick your hand in fron of the engine behind the radiator and tell me if you feel a gust of air going through.

Thanks, i've checked as you said, the fan is not stuck or at least is not on constantly.....any ideas ? can't feel any air.

Then it must be the thermostat, feel all the cooling hoses...they should pretty much feel the same temperature.