Shower off combi running hot and cold

[EmmaMill]

I have a combi Halstead HE 30 in my loft, the past few days the gravity fed power shower runs between hot and cold

The sink hot water taps are ok as far as I can tell, I have mixer taps.

I look at the boiler code as the sink taps run, it stays on H, but when I use the shower, it switches between h and H every 20 seconds.

Any ideas what could be causing this problem? Thankyou.

 

[ShaunCorbs]

As no another taps have this problem I would suspect the shower as it’s a pumped shower on a combi system