i use sram chains on my Dh bike, the outer link plates are v.slightly thicker & the chains are sronger. shimano's will shift better though. especially when new.
the only thing i can think of, is maybe your casstte has a twisted tooth. hard to spot as they are from new, (hyperglide) in the past ive used a brand new one of the shelf, to compare.
as you said your's is new, but you've had this prob. then maybe, due to very slight misadjustment &/or not the best com. chain, putting power down straight away, (first ride) is what caused the pot. prob. (as above)
blimey, its to easy to get carried away trying to diagnose probs. its like being at work, exept ive got a bloody hangover, sat at home
89' Ridgeback 602lx/603
91' Muddy Fox Alu Pro
87' Muddy Fox Courier
86' Muddy Fox Courier
85' Porsche 924!
On the Kona hunt ..