What BB are you using? It could possibly be a flimsy BB but not likely as even the cheapest versions are usually rigid enough. Have you check if the chainrings are straight, bolts tight, crankarm bolts tight on the BB axle, any play in the crankarms? Are you running the correct front derailleur for the groupset speeds i.e. not running an 11sp derailleur with a 7sp chain which may be a little narrow for the wider chain. Frame cracked? Some frames are just flimsy and will rub no matter what but not that common.