Just built up a NOS carbon mongoose frame.

Here is a weird one. When just pedaling smoothly, no chainsuck at all (I acknowledge a new chain and a year old middle chainring off a different bike).

When I get on it (and that frame flexes... ) chainsuck.

I've got no problem trying a new chainring, figured that might be a problem. BUT... I'm wondering if I'm pulling the chainline so out of whack that that is causing it.

XT cranks... with the soft middle ring.

Just wondering others thoughts on this one. Usually old chainrings just suck whenever I've found new frame, chain, old crank combos.

any thoughts?