Fluctuating cold water pressure

[Lukebarney]

Can someone help?
The problem started with my shower temperature fluctuating from very hot to warm and then back. It is a Mira 415 shower run off a combi boiler. At first I thought that the shower or boiler was at fault, but then I realised that the flow to all the cold water taps (and also the boiler) in the house fluctuated while the taps are running.
Does anyone have any ideas what could cause this and how to fix it. It has only just started happening about 1 week ago and my neighbours don't have the same problem so I don't think it can be a mains fault.
Suggestions gratefully received.
Thanks in advance.

 

[cross thread]

you may have a pressure reducing valve on your cold main,this may be playing up

 

[Lukebarney]

Thanks for the suggestion Gordongas, but there is only the stoptap in the kitchen with a tee straight to the sink tap one way and the rest of the house the other. Sometimes the flow is perfect. Other times the shower goes hot and cold every 30 seconds or so. I'm baffled!