Joined: May 2008
Posts: 10,106
Likes: 2,762
From: Fredericksburg, Va
Bikes: ? Proteous, '65 Frejus TDF, '73 Bottecchia Giro d'Italia, '83 Colnago Superissimo, '84 Trek 610, '84 Trek 760, '88 Pinarello Veneto, '88 De Rosa Pro, '89 Pinarello Montello, 'Litespeed Catalyst'94 Burley Duet, 97 Specialized RockHopper, 2010 Langster
Based on your initial description, I would suspect the BB. It is load dependent and at the same location in the stroke. If not the crank spindle interface, then I would suspect the BB with having a ball or race with damage. However, the later could be random depending on the rotational characteristics of the bearings.
I believe you can replace any ball and cup BB with a sealed bearing unit. The key is having the right threads for the BB shell. Don't forget to measure the spindle length and if it has offest.