Hi, I´m sorry to tell you that I had the exact same problem.
I changed the thermostat, the coolant cap, the coolant reservoir. When my car was running I noticed a bubble coming out the coolant reservoir.
When I stopped the engine, the coolant on the reservoir was totally consumed.
I´m sorry to tell you that it was the head gasket blown. Apparently it was a minor leak that produced some gas bubbles into the cooling system provoking the coolant to boil at some points.
You should not worry about it, the gaskets kit is not too expensive (around 100$) and after they fix it it was running just fine.