hi folks need some advice

i recently purchased my old 1993 impreza back after it had a lot of work done to it. it has just had the engine rebuilt so i put 400 soft miles on it with the boost screwed down to 0.5 barwith the intention of screwing it up once run in. i have been driving the car for a week with no problem at all untill the other day, suddenly the car will cut out as soon as it goes into positive boost it starts and drives perfect in negitive boos but as soon as the car tries to boost its like fuel cut so i let off accelarator and it runs fine again untill i hit positive boost. i think it may be a sensor but im not to clued up on all this stuff anyone got any ideas what could be causing this

cheers.

 

could be to do with the boost screw. boost been cut