Pull the chain off the chainrings and lay it on the BB shell. Then, turn the crank without the chain to determine if it's in the BB or is chain related. If it's in the BB, remove the cranks and turn each bearing by hand. If they turn smoothly, the bearings are fine. If they don't, that's your problem. Replace them. If it's chain related, see if you can narrow it down to specific links. Does the RD "hop" periodically? You might have a link that's distorted or has been twisted so that it doesn't move freely.