Shower water feed keeps stopping?

I have a shower cubical and until rececntly the water ran fine. I had a mixertap replaced in the kitchen and ever since the water pressure in both of my showers has been irratic.

I can turn the water on and it's fine, then gradually the flow can get slower and slower and eventually stop all together even though the tap is open. If you then take the shower head off the holder and lay it on the floor of the cubical it then starts to trickle again but as soon as you lift it up to shower under it it stops.

What could this be? As all the feeds to the basin taps in the house are fine. It's happening to two seperate showers in the house. When you turn the water on it does splutter a bit and I'm guessing this is air? If this is air in the system how do I get rid of it?

Any help greatly appreciated, I love my showers!