I'm new to all this so please bare with me...

I'm having a problem with a combi-boiler and need help trying to
identify the problem -

The boiler appears to be working fine - i.e. it's able to heat the
radiators and water. However when heating water for the shower it seems
incredibility sensitive to changes in water pressure, if someone were
to turn the tap on in the kitchen the hot water flow in the shower
completely stops. Also, if the heating is on it seems to frequently
cause the hot water in the shower to fluctuate.

I'm wondering is it possible to control the boiler so that it gives
priority to heating water? Also, is it possible to prevent the affect
of turning on a tap in another room?

TIA