Shower, Boiler, Strange Hot Water!!?

Please help! I have a combi boiler, mixer shower and a kitchen and bathroom sink tap.

If you turn the kitchen sink on hot the water comes out scalding hot. If you run the tap in the bathroom sink, its scalding hot. If you run the tap in the bath have a shower it only comes through luke warm!! The hot water pipe to my bathroom feeds the sink then carries on to the shower then the bath taps.

If you turn the sink on in the bathroom and feel the pipe it gets scalding. If you turn the shower on the water pipe between sink and shower runs scalding for a few seconds then goes luke warm. If you turn the sink on again it becomes scalding again!! It doesnt help to have them both on at the same time.

It seems to me that the shower is not sending a message to the boiler to fire up kind of thing? Although i spoke to a corgi guy and he said if it was a boiler problem then wouldnt even get warm water to the shower??

Please help I had 3 "experts" round now and not one of them has any suggestions - the plumer says its the boiler and the boiler man says its the plumbing!!

I'm quite handy myself so i've tried to figure it out but it just doesnt make any sense!!

Help!!

Thanks

Helen