Baxi 105e hot water problem?

I have a pain of a problem with my Baxi 105e. It’s not quite 4 years old, the problem we have is as follows. The heating works fine, and it runs hot water. But when running water, after the initial full flow of the gas it switches off, then realises the demand is there again and switches on full flow, and off again when it should reduce down. The outcome of this is a luke warm bath! Our heating engineer who originally fitted it initially thought it was the divert valve, which is now not the case. Then the flow charts took us to the gas valve, which we changed to no avail, After contacting Baxi they suggested the PCB, which we changed to no avail! It’s now had 3 new parts and we’re now having scratchy head time