This has been a nightmare for me. During the summer my 91 535i used to overheat even when idling and soon, sure enough the headgasket was blown. So i changed the headgasket, the radiator (the neck connecting to the hose was broken), the waterpump (leaking coolant), thermostat, fan clutch...I also had a mechanic check the tube that runs along the left side of the panel, from the coolant tank to the radiator, he said it was fine. Now it doesn't overheat while idling but the indicator goes over the halfway mark while driving.

Does anyone know what the problem might be? Also, an easy way to check it (no money to take it to the mechanic at the moment).