v70 diesel automatic no acceleration

[whitlock]

help! I know v.little about cars and have had problems with my 2001 v70 diesel automatic. Bought last summer with @ 80,000 miles on the clock.
At the end of last year it suddenly wouldn't change up & speed stuck at @ 25mph. If the engine as turned off, all seemed OK, but after a few hundred yards the same happened again. It was taken in for diagnostics but they couldn't find anything wrong.
Happened again a few weeks later, but this time engine cut out. Left it a few minutes & it did restart & it limped home, again at no more tha 25 pmh. This time a valve (?) was changed (they didn't really explain what this meant). All OK for another few weeks, the it happened agin in early Jan - it wouldn't change up & stuck at 25 mph. Again taken into garage & they changed the valve (!) again. This time apparently they realised that when this valve was changed last time, it was a like for like change ...but the original valve they took out was the wrong one & therefore they put the wrong one in again! I am told that the right one is now fitted.
Anyway, 2 weeks on, it now seems OK.
Does any of this seem familiar - apologies if these questions have already been asked ?? I guess I would like to know what this 'valve' is, or are they fobbing me off?
Thanks

[Arno]

I don't know about the valve(s).
But one point may be the air mass/flow counter/sensor (excuse, I'm not native speaker, it can be named by other way), a small delicate device located after air filter (so change the engine air filter regularly to protect also that sensor). In this failure case the engine works rhythmically but hasn't power.
Also I 'm able to point to the fuel feeding system. Eg, if to the fuel line gets air somewhere, then the engine may become forceless.
D5 and (Audi's 2.5) TDi engines in V70 are sophisticated ones of course, lot of (weak?) links on the chain. Old robust diesels just needed pure fuel and lot of pure air.