ok, another update.........
Plugs and coils have now been replaced. Guess what......no fookin difference!!!
I'm out of ideas now. I can't understand what's going wrong. I've changed all the most likely candidates and still the
problem remains. Unless anyone can suggest anything else to try, I think i'll be taking it to a specialist.
Just to recap the
problem, in 5th gear only, 120mph or over (5k rpm), come off the accelerator, car starts shuddering, vacuum pressure goes nuts (not boost) and feels like the car is about die. Once it's coasted down under 5k rpm, it sorts itself out.
Don't suppose it's anything to do with back pressure in the exhaust is it?
Blocked CAT or something?