Just be aware that some of the mainstream servos now have B5 as a default diesel fuel. This may not be the problem but just check when you are filling it. It should be noted on the bowser that it's B5. I'm not a fan.
And, just on your driving, how often do you give the engine full throttle and work it hard (to max revs) under a load, ie., up a steep hill?
JCL's suggestion about a build up of carbon deposits is the likely explanation and is what I was trying to get at when I asked about your driving regime. Ultimately, the problem most likely lies somewhere between your driving regime, your driving style and the fuel you use. It could be outside of those areas but these are the "usual suspects".