you should get very minimal negative pressure on a diesel. The reason you get negative pressure on a petrol, (turbo or not) is due to the engines 'suck' against the (closed) throttle plate. As a doosel has no throttle plate, the only negavtive pressure you will get it against the air filter, which should be negligible.
im afraid ive missed the first part of this discussion, but what engine do you have? Could you explain the symptoms quickly?
*edit* read it now...
the fact that it slows quickly when you lift off suggests to me that you may be barking up the wrong tree... check the obvious stuff, is the handbrake binding? Are any of the brakes stuck? Do you have a stuck UJ?
I don't know TD5s in much detail, but i would imagine it would have a boost sensor. If this is not reading the correct pressure, it may well give an over boost error, and/or may not realise that you are on full boost, and so not add the extra fuel it should?
Im sure someone with more TD5 knowledge will enlighten.