We have returned to making repairs and have yet to resolve over heating issues. To date: have replaced thermostat - plastic water pump ( new - metal) - plastic housing that holds thermostat (new - metal) made certain no air in hoses / with each replacement car warms up and holds at mid point on gauge, once underway . . . . heat stops in cabin and temp gauge red lines ???
It does sound like you have a coolant flow problem. Here is description of the system which may help pinpointing the trouble.
The puzzling part is that the heat stops in the cabin. Can you tell whether the coolant is genuinely overheating or not when the fault occurs or is there just an indication error?
Church Stretton, UK