thermostat could be stuck open. 80k miles is getting up there in mileage and a cooling system overhaul may be in your near future. When you travel 40mph or so, there is enough air going through the radiator to keep the engine at normal operating temperature. But since its been super cold by you (-20*F yikes) the X3 is prone to extreme cold weather problems, at least from what I read and hear.
Did you check your coolant level? Check for leaks?
National Capital Chapter #465420
2000 ///M5 Oxford Green
| Caramel Nappa
| Walnut Interior | Luxury Package |///M Registry
| My Progress Thread
2006 X3 3.0i M-Sport | Silver Grey
| AT | Navigation | Premium Pkg | Cold Weather Pkg | AFE |
2000 323Ci | TiAg | Gray | 5MT | SOLD