1997 540i...when I'm driving around town the temperature gauge will stay centered as normal for a while but then it will start rising. When I drive at highway speeds, tach basically over 1800 rpm, the temperature will stay at normal or slightly above. If the temp starts rising if I either rev the engine or hold the idle speed at around 2000 rpm the temp will then go back down. It's winter and prior to this the heater air temp will get cold.

I did loosen the bleeder screw last evening but didn't get any air out, could there still be air trapped? What else could be the issue?

I change the water pump last summer along with the thermostat. My coolant level sensor always tells me to check coolant level but it is at the maximum.

Thank you'97 540i auto