Having just read you entire problem and everyone else's reply, i can confirm that my 4x4 saff cossie is doing EXATLY the same. I have changed everthing, all the sensors, engine loom, ECU you name it. I have resorted to taking the entire plenum housing/inlet manifold off, today and just re sealing the whole thing, it's the only thing left to try.

Does your idle ok, but slow if you dissconnect the ISCV? Mine does, but it does stop every now and then. It's quite driveable, but you cant really cruise along. If i just change down and leather it, it's fine. Try letting yours idle, with the ISCV dissconneted, and take a vacuum/boost pipe of, like the one to you amel valve, and see if the engine changes not or anything, because it should and mine doesnt, so im going down the "air leak road" I'l let you know if this cures my problem when i have done it.
Bye the way, the plenum is a pig to get off!