We have a Rheem low pressure 135L hot water cylinder which uses a pressure relief valve and a 7.6M pressure reducing valve, model 14T.135.13, max pressure 7.6mtrs, and the problem is with the shower where if you have the mixer at a certain spot, I think were it is outputting to much cold, then the pipe in the Hot Water Cylinder cupboard makes an awful knocking sound and water starts spewing out of the pressure relief valve. Does anyone know what this is most likely due to, I was thinking it could mean that the pressure reducing valve is faulty? Should I try replacing that, as I can get a new one quite cheap and then try installing myself? Or could it be the pressure reducing valve or even something else?
I did get a plumber in originally ages ago but he couldn't fix it and gave up so I have been put off getting another plumber and just lived with it as we can still use the shower fine so long as we don't start the shower at that certain spot but we find guests always mess it up so I need to get this sorted once and for all.
Any feedback would be much appreciated.

Linkback: https://www.plumbers.nz/q-and-a-hot-water-cylinders/44/hot-water-cylinder-and-shower-issue/2002/

Yip, agree with Robbo, sounds like the issue is with the shower not the limiting valve.