[Closed] Trek 8500 chain suck

My friend has got a Trek 8500 and was suffering awful chainsuck on a ride in the peak this weekend. The bike is almost new, so the drivechain is in good nick, and the chain is regularly degreased and lubed. I checked the chain for tight links, checked the front mech is correctly positioned - I can't think of anything else that can be causing it!

I've got a geared inbred (notorius for chain suck), and I had no problems whatsover! The only difference is I'm running Deore steel rings and my mate is running XT aluminium. The chainstay clearance is pretty tight on both bikes.

Anyone else got a Trek 8500 and suffering similarly? Any solutions? (I'm thinking maybe a change to Deore rings is in order...)