High temp reading

I have a 2000 Oldsmobile intrigue with roughly 125k miles on it I am having troubles with the the temp gauge going beyond the half point mark when i am at highway speeds or the AC is on i brought it in and the head gasket tested good along with the thermostat and my cooling fans and there is no leaks and has proper pressure, Now i have heard there is an air damn underneath the car that direct airflow how important is this could this be my issue also i have heard switching out of the dexcool cause its so acidic and always gums up is this also true im kinda lost since i have gone down the list with test and all has tested good some one help please before go out of my mind