Overheating Problem on a '86 Ford Econovan 1.4 Petrol (116,000 KM)

I would very much appreciate any help with an overheating problem I have with a '86 Ford Econovan 1.4. First noticed the problem 24 hrs
after buying the van. After driving for 1hr around town (including motorway driving +90KM per hour) the temp gauge went above normal and close to the red. Water appeared to be boiling in the reservoir and in the main feeder pipe from the radiator to the engine block.
The van has been standing for 2 years without being used, so once the water had boiled I have done the following:
1, Checked the Thermostat and replaced it with Ford Original Spare. 2, Removed all connecting pipes (including the cab heater pipes) flushed and cleaned the pipes. Ran water through the engine and flushed water through the radiator. Flushed the whole water-cooling system with a "cooling water cleaning fluid", twice. 3, Checked the fins on the water pump, which seem to be complete and rotating. 4, Replaced the whole system filled with 50% Water 50% Coolant, ran the engine to warm, parked the van on a hill (front of the van up the hill) took the cap of radiator to remove air.
Still OVERHEATING, "Boiling", after 15 min driving (high and low engine loads).
When I run the engine without the thermostat the engine temp seems to run OK. A little high, depending on engine load.
I've run out of things to check, as the water system is not rocket science.
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