Hey Kenneth,
From an autmotive engine reconditioner & mechanic's perspective ( in a previous life), in no particular order.
Blockages/flow restrictions in radiator.
Blocked or restricted radiator airflow capacity.
Coolant flowing through radiator to quickly ie removal of themostat, if this was done recomended just cutting the centre out of the themostat so water was slowed sufficently to cool in radiator before rentering engine.
Pressure loss, obviously causing some coolant loss but also not maintaining the pressurised increased boiling point, see also radiator cap not holding prescribed pressure.
Combustion gas entry into cooling system, can be detected by a TK Dye check which changes colour in the presence of CO.
Cooling system capacity not meeting engines cooling requirements. Commonly seen with modified engines with significant performance increases necesitating larger radiator & better radiator airflow.
Stupid things like blocked/restricted exhaust, too lean mixture, incorrect ignition timing.
As was mentioned above water pump cavitation, generally caused by either corrision of water pump impeller or corosion of area where water pump impeller runs ie to larger a gap or too much tubulence.
Themostat issues, jamming not opening at prescribed temp, openning intermitently.
Cheers
Stewart
