I have an 81 300D that whenever I drive over about 60mph for 10 minutes, the temp gauge steadily rises from it's normal position at 80 to around 90 without the a/c on. Is this considered a "normal" range? I know redline is 120 but could my thermostat be functioning incorrectly? If I reduce speed to 30-35 the temp drops to about 83 or so, and even sitting in traffic it's hovering around 85. I haven't heard the aux fan come on, but that may be only related to the a/c. Any suggestions?