Electric shower temperature fluctuating

[Subby]

Hi all plumber here...
My customer complained about fluctuating temperature at the electric shower so I removed the said shower and replaced with a new one.
They still have the problem, and a neighbour has suggested it may be "something to do with the fuse box"
Is it possible that this could be an electrical fault somewhere?
The rcd is fourth from the left (next to the one labelled rcd circuits) the shower is 9.5kw. Nothing trips out.
Thanks in advance.
I tried to upload a picture of inside the consumer unit, the live wire to the shower looks to be about 8 mm diameter and the shower is about 4 metres away from the rcd.

 

[Dave-geo]

Make sure they haven't set the temperature too high. Most showers have a thermal cut out that turns off the elements when the temperature reaches 47 degrees. If the temperature is set too high the unit will cycle hot and cold. When it goes cold the obvious thing to do is turn the temperature up which only makes matters worse.

 

[Dave OCD]

I'd say there's a fair chance it's the pull switch on the way out, if you look inside [with power isolated] you may see signs of overheating on one or more of the cable cores near where they terminate in the switch. A fairly common problem caused by poor installation or carp switches that aren't really up to the load that runs through them.