Hi,
It could very well be a bad 'T'stat, or a leaking Coolant Reservoir, or bad Spring Clamps, or even a leaking Hose.
You'll need to do some investigating to zero-in on the cause.
A leaking Coolant Reservoir won't necessarily leak all the time. Small cracks may open up (expand) when the system is Hot and re-seal (contract) when the system is Cool.
Same thing with a failing Spring Clamp. System Hot (pressurized) the Clamp may allow Fluid/Steam to escape, while Cool (unpressurized), it seals OK. The same goes for leaky Hose(s).
The 'T'stat can be removed and checked by submurging it in Boiling Water. Check that it opens. For accuracy, use a Thermometer and see what temperature it opens at. But, for the 'T'stat, since you need to replace the Gasket, etc. when removing it for a check, on a car with some mileage, it's best to simply replace it altogether.
You can get a System Pressurizer:
http://www.tooldiscounter.com/ItemDisplay.cfm?lookup=KDT3700 and pressurize the System Cold. Then you can check around the system while it is pressurized to look for Leaks. The Pressure Tester @ $90+ (possibly cheaper on eBay, Craiglist, etc.) may seem expensive at first, but it will allow you to do most Cooling System R&R yourself, eliminating expensive Shop Time - a worthwhile investment which you can also use on all your other cars as well. Hope this helps...
Happy Motoring!... Jim'99
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