Are you not meant to use a thermostatic mixer shower with a combi boiler system, rather than just a normal hot& cold mixer which is sensitive to pressure/temperature fluctuations?
We've just moved into a house with the Baxi Bahama 100 combi boiler. Previous owner was plumber and boiler/heating/plumbing system about 4 years old. I would think the system has been regularly serviced. In fact he muttered something about "well it works NOW" as if there had been problems...