hi mate had a similar problem with my mates fiesta turbo, it would start and run fine but as soon as you tryed to hit boost it would try to cut out and generally jump about, had a good look at it and it turned out that the crank shaft position sensor wiring had corroded and when hitting boost would cause the engine to tilt backwards pulling the wires apart! soldered in a new bit of loom and a new multi plug and has been sweet ever since could be worth a look.