Mixer tap shower fault !!!

[Systemfix]

HI Guys,

I've got a problem with my shower, it's a bath mixer tap with a hose connected to it to the shower head.

When its running the pressure is ok but it runs hot and then cold and then hot again and cold, it keeps alternating and can't figure out why?

I have thought of the cause to be the hot water cylinder but cannot be sure.

The rest of the taps in the house are fine in the kitchen and basins.

Any pointers would be a great help,

Thanks