I am not mechanically minded but if it is over heating I would say that the thermostat is knackered.
When I switch mine on they build up to pressure and then over say a 15 minute period I get a reduction of steam flow as the steam reduces and water is added to the heating cylinder, which then repeats the cycle.
After the initial warm up they are quite quick to then heat up.
Are you getting steam. how long is the steam lasting??
Rob