V70 D5 manual loss of power.

[monsterv70]

Morning folks.
I can’t see it as a sticky so thought I’d ask although I’m sure it’s been asked before.
I have a 2006 D5 manual estate with 90k on the clock, owned for 8 months. Since I’ve had it I occasionally get a repeat ‘engine system service required’ message. It comes on if I give it hard acceleration or perform something like a motorway overtake. The power drops, not dramatically but enough that you have to change your driving style accordingly. The light remains for about 4 subsequent journeys before going out and power feels restored, until driven hard again.
I have learned to drive Roy d the issue as I drive at very quiet times of the day so do t need the power. It’s bloody annoying though and needs fixing.
The car returns excellent t mpg, does 80+ miles a day to work at motorway speed, doesn’t smoke at all, ever, it’s insanely good other than the fault.
I had the codes read (I wasn’t told the code) and was recommended by the mechanic, a friend of a friend to replace the air flow sensor. No difference made. I have replaced the top thick rubber hose from the top of the intercooler which helped a little but it still persists.
It feels like it is being starved of air or similar.
Any suggestions before I start having to Chuck money at it? It’s an old car and not economical to throw loads of cash at it, but it is a bloody good car other than this fault.
Thanks in advance.