Mira advanced ATL shower works like on/off

[Septemberman]

So I'm standing in the shower having a wash ad you do when all of a sudden the shower starts operating in an almost off almost on constant sequence which became very aggravating because the shower also became very slow so it was like a slow pulsating wash with bursts of cold air waiting for the next lot of water every 2nd minute.arghhh.someone help

 

[Sharpend]

Pump? 
loose connection? 
water blockage?

any strange noises? 

 

[binky]

someone flush a toilet elsewhere in the house? Dishwasher / washing machine taking water? 

 

[NozSpark]

scaled up head and/or hose?