Liquids will have a higher boiling point under pressure - with a leak in your system, the coolant will boil at a lower temp. A leak could very well cause overheating.
While you're at it, you may want to check out your thermostat - for such a simple device, it can cause a lot of issues if it isn't working properly (e.g. stuck open or closed - closed would definitely end up with overheating).
FWIW, I had kind of the opposite problem last winter - the vehicle was not reaching proper operating temps and threw a code- problem was the thermostat was faulty and stuck open.
Hope you determine the problem - overheating your engine will definitely cook it (and your pocketbook) in the end.
'03 Ranger Supercab, Flareside, 4.0 liter V6 SOHC, auto, 4 x 4 w/FX4 Off-Road Package (Level I)