sounds like bad luck as you sound like you know you're at, but for others reading, hows the indexing, any crunching sounds going across the block? is the front mech too low, ie: rubbing on the middle ring? how are all the cogs, replacing the chain, good, but are the teeth worn? is the cassette locked on the freehub tightly? are the cables too taught?

i just refitted all my drivetrain, i'm no mechanic but provided the bb axle is correct for the bike, mechs are in the right position (& not bent), cables are lubed & pulled hand tight (not overly tight), the chain when on largest / largest should be tight THEN ADD TWO LINKS, angle of the dangle screw is not making the rear mech do nasty things, rear hanger not bent, all links on the chain move freely & are cleaned / oiled, etc etc... maybe other things too

or silly things like, 9sp hub with 8spd system?

sounds like something is too tight somewhere