Slipping when pedalling
Drop the chain and try to rotate cranks. If "skip" feels then I would look into BB. If not, then pretty much sounds like kink in the chain which is very possible on very new or very old chains.

Very new chains can be over-pressurised while installing and therefore one link connection is just not flexible enough which makes it "skip" when going through the derailer. Old chains have rust and dirt problem which causes same problem.