My pumped power shower is only giving hot water and cuts out completely at low temperature? Help!?

When I turn the shower on it starts to run before it then cuts out - it has worked fine for several months (we just moved in) - but now when I turn the shower on it cuts out after about 5 seconds; it comes back on again but only at high temp suggesting that the thermostat is malfunctioning. Any suggestions as to... show more When I turn the shower on it starts to run before it then cuts out - it has worked fine for several months (we just moved in) - but now when I turn the shower on it cuts out after about 5 seconds; it comes back on again but only at high temp suggesting that the thermostat is malfunctioning. Any suggestions as to what to try welcome!
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