yeah i dont blame them, why a belt would slip during a shift, and only during the 1-2 shift, and not when youre just free-revving or when cold would baffle me as well, especially if the belt is in good condition and the tensioner is working.
are you absolutely positively sure its a belt and not the intake making a whistle that sounds exactly like a belt slipping? i had a dsm, and the ps/alt belt slipped all the time, new belt, alt, didnt matter. its a dsm thing, and i know what it sounds like, its a sound file tucked in my head that basically played every day for a couple years, and when i installed the intake on my GT, it sounds just like that when its warming up, and when i take off from a stop between 8-1100rpm at light throttle. freaked me out the first time, but i kinda like the whistle now, guy asked me if its my vortech sq(super quiet :dunno
supercharger in the mall a while ago cause he heard me take off from the stop.