Another thought, there may be a pinhole leak somewhere (you can hope) put a good quality "stopleak" in the coolant system and see if that works, try the cheap fixes first before you move on to the more expensive ones. As Woko says it could be the thermostat, but that is a bugger of a job to get to because you have to pull the manifold off the front bank to get to it.

I had similar symptoms, tuned out to be the water pump.
Just leaking out slowly and evaporating.