help with plumbing a shower

[dts]

cct, is the pressure and temp dropping when another tap is being used ie coldtap as this would drop the cold feed pressure from the boiler which does cause these symptoms in most combi boilers.
As of your prob frogeye sounds like you have a leak on your system which would drop the pressure on the boiler and finally the boiler shuts off

If you have anymore probs cct email me and will try and help further

Goodluck

andy