To me this sounds like the stereotypical Bottom Bracket / crank interface although I have not wrenched in ages other than my own bikes now.
I use Campy and have UT and threaded english BB for 2 bikes. On one bike I have no issues (I measures everything before installation, etc). On the 2nd It has the exact same >200w / 80rpm clicking noise which gets worse at higher torque times. If I double pedal I can minimize it. Light smooth pedaling .. nothing. heavier pedaling .. clicking.
With Campy UT it's a fixed width so one can calculate stuff out, although a major PITA.
But someone mentioned hambini. he has identified that the two sides of a BB interface may not be perfectly directly across from each other, thus causing the crank/axle to be slightly skewed in relation to the bearing/cups, thus causing what seems the correct installation to not really work no matter what you do.