how much boost is your car running? if youve got more boost than the standard ecu chip is limited to, it will do this. will boost higher in lower gears but when under load the boost cut will kick in. you say it boosted fine afterwards, was this in 4th or 5th? assuming it was (and the car is chipped to allow more boost and set up accordingly) then i think i may know what it is. i have a similar problem and it only occurs after the car hasnt been run for a while. once its been up to temp for a while, it behaves. its damp related and i think its moisture affecting the dizzy and/or hall sensor (the plug on the dizzy). the vac that goes to the ecu is a good shout though