Don't know if its related but I can tell you about drilling a small hole in the thermostat - perhaps.
Many moons ago I had a Cortina 1600. All was fine until I had a hose leak. Simple fix you may say. Well yes but after that the engine would warm up, get too hot and then boil - if it got past that it was OK. Thermostat (checked opening with thermometer) and fan all OK - it took me weeks to sort it even got new thermostat. Decided it was air in the system after even after bleeding and despite best efforts couldn't stop it overheating on the way up to temp.
Looking at the castings I decided that air trapped in the t/stat housing was the problem. Or, for some reason the thermostat sensor was too far from the block so wasn't being heated fast enough to stop over heat. Solution - drill a very small hole in the thermostat to allow air and a very small flow of water to bleed through - Problem Solved forever.
Funny but some months later see neighbour out under the bonnet of his Escort (same engine!) got the same problem - drilled a small hole and sorted...he thought I was a genius.
So small holes drilled in thermostat - probably the same reason...??
