Friends merc went in to get a new thermostat fitted, since then it's developed a misfire, generally starts on 3 then goes back on 4 but once warned up it's drops one again, there is White smoke coming out the back and lots of pressure in the system when taking off rad cap,so I would say head gasket although after a compression test it's got between 145psi-160psi, I don't have a block test for gasses but wondering could it still have good compression cold and warm if the gasket had gone?

CheeRS
Craig