my series 2 used to suffer from the same sort of thing, holdind back asif it had lost all boost pressure i took it for a setup and they found that when you disconnected the knock sensor on the inlet it ran and boosted fine but if you reconnected it it would boost so much then it would retard the ignition. they said there was a problem within the ecu on the knock sensor circuit and they had seen it loads of times before. there answer was for me to find a series 1 ecu and they swapped my chip over for me and tried again, never had a problem since as the series 1 doesnt have the knock sensor and now she runs like a dream. just a thought dont hold me 2 it but if your stuck its gota be worth a shot.