Charlie,
had a simular problem this spring with my truck. After doing a bunch of different things, it ended up being the constant voltage regulator behind the speedo, replaced it and it was very accurate after that comparing it to a borrowed inferred gun I used. But in your case I don't think its that if your able to put your hands on it without getting burnt. One thing too Steve we used to do in Wisconsin growing up insted of cardboard, we'd just take the fan off all together, just wouldn't let it idle too long!
Michael
MichaelSandy Valley, NV