It's possible the head gasket could have been weakened from the first overheating episode. Any time an engine overheats you MUST pull over then and there and shut it off. If you continue to operate the car you may be turning a minor problem into a major one.
The mechanic should have at least made you aware of this the first time.
The thermostat could also be responsible for this and that's a quick, cheap fix.
As to any head gasket failure (assuming the HG is even bad) that's a touchy issue. Chronic overheating and continuing to drive the car only causes more problems involving piston rings, valve seals, etc. In other words, replace a HG and you may have a car that burns oil quickly.
My opinion is that a compression test should be run on all cylinders and if a piston ring problem exists (and if the timing belt has never been changed) it may be time to dump the car or replace/rebuild the engine.