Timing Belt

[BW2309]

Air lock very probabale, if the air lock is around the coolant emp sendor for the fan (bottom of the rad) it's measuring air temp, not water and so wont give the right reading. This means fan wont cut in at correct times and the air lock itself causes loss of flow and hot-spots.
Check the flow rate by removing the return to the expansion tank and revving the engine to about 2500rpm briefly. You should get a good solid jet of coolant out of it. If not, there's a blockage, stuck thermostat or air lock.