My shower gets too hot.

This could be a faulty Stab valve however carry out these checks first!

The things to check first are. 

1 That the main valve coming into your house is fully open. 

2 Then check that the valve on the line to the shower is fully open. 

If these valves are not fully open then there may not be enough water 
getting to your shower to cool it properly the overheat safety device will turn 
the power off until it cools Then it all starts again

Make sure that the shower head is clean and clear, that the hose isn’t kinked (this can happen just behind the hand set where it is difficult to see) also check the inside of the hose although it can look OK on the outside it could have collapsed internally 

The other possibility is a faulty flow/stabilizer valve not allowing the correct amount of water into the shower. 

A ruff check is to remove the shower head turn temperature control to fully hot the water coming out of the hose should be a bit more than a dribble then turn to fully cold and it should be coming out at full flow. 

If there is not much difference between the two then you could have a faulty flow valve. 

Carry out this check after you check the inlet valves.