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Sounds like you may have two problems

First the reduced temperature.

 It may be a faulty element in the heating tank (there are two, three in most 10kw showers) you can check them with a multi meter you should have a reading of around 12>18 ohms.

If you have power to the elements and no heat it's probably the elements, no power to the elements then it's probably a micro switch.

http://www.youtube.com/watch?v=wzPt1hsfVq4

If you don’t have a multimeter  we sell them at £10 each with a guide on how to use them for checking and testing the various parts of your shower.

Apart from helping to fix your shower, they are really useful for checking you have a power supply to appliances, for checking fuses, letting you know much power you have left in batteries

(so you know if they are dud or not)  and lots of other jobs, in short they are well  worth the £10.

http://www.showerdoc.com/installation/electrical-products/SKYTRO-600-005-Digital-Multi-Tester

And the cutting out problem.

It may be your solenoid valve, you can check it if you have a multi meter, you should have a reading of at least k3.5 ohms, across the two terminals any less and you need a new valve. (Check the valve in the failed state.) (When it has stopped working!)

If there is power getting to the solenoid and it is not operating then there is a good chance that you have a faulty solenoid.

http://www.youtube.com/watch?v=NzJjzWqxesc

The video is intended as a guide and does not cover every shower.

If you don’t have a multimeter  we sell them  with a guide on how to use them for checking and testing the various parts of your shower.

Apart from helping to fix your shower, they are really useful for checking you have a power supply to appliances, for checking fuses and letting you know much power you have left in batteries

(So you will know if they are good or not) and lots of other jobs, in short they are well worth a few pounds

http://www.showerdoc.com/installation/electrical-products/SKYTRO-600-005-Digital-Multi-Tester