tbh it could be easily creeping to fuel cut, but with fuel cut its like some1 slapping the brakes on but only lasts a second or so, not what hes describing. the things i can think of is clutch slipping? and the most likely thing would be a boost leak is just like you described reving up as normal but not pulling, also try and borrow some ht leads,i had a problem with mine recently and it was really sluggish!! hope you get it sorted