Inconsistent shower hot water

[oak orchard]

Hi ... The shower is delivering inconsistent water temps. Sometime its fine, sometimes it goes cold and has to be throttled up. The kitchen, utiliy and vanity DHW supply temps are not affected by this so I am assuming its not my boiler.

I have a tank in tank Triangle Tube boiler driving heat and DHW. All fixtures are connected to a Maniblock.

Is this problem likely to be a failing Mointrol mixing valve?

We are on a well and can get hard water deposits building up in plumbing valves.

Thanks