Originally Posted by
operator
Improperly set b-tension will do this.
I'd say it's got to be the b-tension adjustment.... But, the OP said the bike was originally a 6 speed but is now an 8 speed. If they were spread the chain could be clipping the taper of the chainstay and jamming. I've had 126 mm spaced bikes where the chainstay protudes more to the inside than on most modern 130 spaced bikes.