Having finished the uphauling of my 2003 135 TF now I have found that the head gasket is shot. It was probably "treated" with some sealant that hided the problem, but after changing radiator, PTR therm, water pump and some pipes and thoroughly cleaning the rest of the circuit, all the signs of HG failure have appeared in full bloom.
We have also discovered that the PTR had been bypassed and, as the PO didn't know anything about it, it seems that the engine has run too cold for a number of years. Adding to that the 75K miles it has on the clock, I have decided to bite the bullet and go for a new, reconditioned stripped engine.