[Vwdiesel] coolant questions

[Pam & Doug Boes]

I had a high temp indication and found the reservoir empty. I filled it and
still had a high temp indication. When I checked the reservoir it was under
pressure after sitting all night and blew coolant all over the ground when I
took off the cap. I siphoned off some coolant to get the level below the
return line from the radiator. It was real cold the next night and the gage
said it was running hot again. When I checked he coolant, it was gone from
the reservoir.

I filled it again and started it and there is not immediate pressure, like
I'd expect from a blown head gasket. It this how a dead thermostat works?