Sorry, haven't read through all of above closely enough so could be blowing smoke but could it actually be the cassette hub that is slipping? I have had an old cassette hub slip in the past (geared not ss although that is not really relevant).
Just a thought, probably a pointless one.
i was reading down the page and the gonna say freehub body failier if it ain't the tensioner you beat me ed
Colnago master blue mapie art deco 2001
Diamond back ascent full xt
Saracen killi team 1998 and 853
Lots more on the goThe Gates ride - 27th July 2014Herts Epic ride - 16th August 2014