My interpretation was that the 50t chainring is rubbing the stays, when under tension from the chain. Campy UT is designed for road machines, and I imagine the extra chainstay width for CX clearance is giving the OP problems.
I have a Veloce UT crankset on my road bike. It's an outboard bearing cup system, and the half the axle is attached to each crankarm. the two halves have interlocking teeth, and are joined by a 10mm bolt. It's wonderfully stiff, but is very picky about BB shell width tolerances. In fact, I'm pretty sure that my previous cup spacer suggestion wouldn't work because of this.
To the OP, you might try a X-post in the Mechanics forum.
EDIT: Nevermind, I see you already did that...
Last edited by justinb; 05-01-07 at 01:46 PM.