yet another tdi power problem

[budd]

I’m having a problem with the wife’s ’98 110 TDi, basically a loss of power under load.
I don’t actually drive the car much but when I drive it any distance I always come back vowing to get to the bottom of it, I generally have a little tinker with it then move on to something else, it doesn’t bother the wife ones driving is largely commuting and at low speed the issue doesn’t arise. Anyway we’ve just got back from a trip up to the north east and it’s yet again I’m looking at curing the problem, only this time I really need to follow it through.

My problem is a intermittent loss of power, driving around town or in slow traffic the car is usually fine revving freely with normal power, but under load (motorway overtaking or inclines are worst)it seems to die not cutting out but it’s as if the power as been halved, which is very frustrating particularly as may usual car is either a Impreza type RA or a TVR Griff 500 neither of which as any power problems!!

Previously I’ve replaced the Maf, cleaned the (very bunged up ) EGR and replaced all the vac hoses, all of which made no difference, so I’m now looking at the N75 valve, which comes up regularly as a cause of poor performance.
My question relates to the function of the N75 valve , my understanding is that it controls the boost pressure and therefore power output, now if my valves is broken then the problem would surely be permanent and throughout the rev range rather than intermittent and apparently engine load sensitive?
Before I buy a N75 valve I’d like to be at least sure I’m on the right track, I don’t want to be endlessly fitting various valves or sensors hoping to eventually drop on the right one, also is it possible to fit a electronic boost controller and manage the boost that way?
I’m pretty much a novice re VWs so if any of the above is wildly inaccurate please feel free to correct it and point me in the right direction and hopefully I’ll be able to rectify this annoying problem.