I just checked and it does happen when using either chain ring up front. So, to sum it up, that's small sproket in the back (campy lock ring on a campy cassette on a campy hub) with either chainring in the front.
There isn't any problem shifting into or out of this gear, so the limit screws seem to be set right.
The jump happens multiple times per pedal revolution. This is a new chain, so chain wear isn't a problem. When pedalling backward in this gear there are a number of places where the rear derailleur moves downward a couple of milimeters. This could mean there are a number of tight links, but hand checking them doesn't yield anything obvious.