Boost cut

This is actually my friends car I just help him work on it it is a 2000 GTP with a 2.8 pulley on right now it is intercooled. When the pedal is mashed around 95mph the boost drops about 5psi almost instanly. If we do it over and over it will do it around 90mph. The air/fuel remains the same when this happens. We had a bigger pulley on the car a 3.4 i think it was and it still did it just not as dramatically and at a lil higher speed. Its really killin us when we race. He got a computer from the grand prix store I personally think it was a waste of money i think a dyno tune would of been better spent money. I just think the computer has somethin to do with this cut but I'm not familar with this engine. Thanx for any input you guys have.