Boiling up

[Enoch]

Here's a really daft idea - have you changed the routing of the water pump drive belt? If so is the pump turning the right way? If it was forcing water back against the wrong side of the stat then it would overheat with the stat in but would probably be ok with the stat out. By the same token - is the stat in the right way round?
Might sound daft but thought it worth asking.
Enoch.