Can anyone help me fix my Baxi Bahama 100 boiler?

Hi, my Baxi Bahama 100 boiler really has me flummoxed. It will provide central heating no problems, and hot water just fine from the kitchen tap (located right next to the boiler), but when you try and run hot water from the bath / shower, is sometimes cuts out (rather than locks out). This cutting out means the water goes cold and remains cold until you turn the tap off and back on again at which point it re-fires and provides hot water again, then the same thing happens. I have a homeserve policy with British gas and the engineer has so far:

1. Changed gas pressure - initially we were getting a low gas pressure reading but this was fixed and problem is still the same
2. Checked water pressure / flow rate - this seems fine
3. Changed fly switches in the boiler - same problem
4. Checked filter in boiler - no blockage
5. Changed heat exchanger in boiler - still no joy

He can't fix it. Has anybody else had this problem, or know what I might be able to do? Many thanks.