coolant level

[TexasCoder]

Let us know how the flush/fill goes (This would be a good time to swap out the thermostat), I would also do a vacuum test on the system. These systems are designed to never leak, and if yours does turn out to have air in it, the leak is coming from somewhere.

It is possible at some point the coolant got low enough to allow air in through the reservoir, but I would suspect the low coolant light would have been on all the time, regardless of driving conditions.

Good luck, and let us know how it turns out!