Why does my shower keep going hot then cold then back repeatedly?

It doesn't matter what temperature I set it on. There are no appliances or other taps being used. No toilets being flushed. It happens no matter what point you take a shower in the day (so I doubt it's anything to do with sharing pipes with the rest of the houses in our street). We are on a combination boiler and a mixer shower. I am no plumber but I'm stumped. Any help is greatly appreciated. I should add this problem is limited only to the shower so I don't think it's the boiler at all...