My s2 rst is doing exactly the same as this, hits 10 psi, holds it, then exhaust rumbles really loudly but car wont go any faster, this is intermittent in 1 2 3 and 4 but permanent in 5th. If i feed the power on gradually, i.e build up the pressure steady, then accelerate hard i can normally get away with it. On about pipes being off there is a pipe off coming from rocker cover? Also there is a bleed valve on there. If it is intermittent, i.e sometimes its ok for a few days then it comes back, surely it must be something on the electrical side?