Chain falling off middle chainring

Ted said:

I have a Campy Record triple crank. Every once in a while while I'm
riding I'll shift the rear derailleur and when I do the chain falls off
the middle chainring onto the small chainring. This only occurs
seldomly and is not easy to reproduce.
I have a Phil Wood bottom bracket on. This bottom bracket allows you to
move it side to side when installing. I'm wondering if it is possibly
not centered right and that this might cause the problem with the chain
coming off.

Ted

Does it seem to happen when you're shifting to a higher or lower gear?
Or does it matter?

It may occur more often when changing from a larger cog to a smaller
cog but I'm not really sure that it matters. I've also had it happen
even when I wasn't shifting gears.

Ted