I had exactly the same I ran round the houses trying everything to no avail, eventually I had one of our mobile techs Jon from Rough luck to look at it, he inserted a camera into the bores via the plug holes and number 1 cylinder was wet, so over heat was due to gasses although a sniff test said otherwise, I also had a multi layer head gasket that han done less than 4000 miles liners were ok, just the gasket had let go, Jon changed the gasket for an N series gasket and all well 4 years hence.