97 Venture overheating when going over 40

I am having a problem with my 1997 Venture. When I go over 40 mph, the temperature gauge starts going up all the way to the red overheating area on the gauge. But if I keep it at 40 or below the temperature stays down. What could the problem be? Is this something I can fix myself?

Possibilities: Thermostat stuck in partial open position, plugged radiator core, low coolant level.