Shower flow starts then stops !?

Hi I hope somebody can help with this frustrating problem...
When turned on, my shower works fine for about 10 seconds then stops and just drips. To temporarily resolve the problem, if you turn on the bath taps for say 10 seconds then off again, the shower starts working fine. Any ideas what can cause this...?
I'm in a converted Victorian house flat and have a Vaillant combi boiler and gravity fed cold (which the tank is about a metre higher than the actual shower rose. The bath taps work perfectly, and the shower feed is fed from these pipes just before the bath taps. Shower worked fine for ages until recently.
Also, I have to mention that when the shower is running normally, the boiler starts 'rapidly firing up and off' which I guess is trying to keep the set temperature of the shower thermostat. This could be a seperate problem but thought I'd better mention it - very loud and annoying!
Hope somebody has any suggestions! Cheers BSW
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