I have replaced my wheels with a Shimano Nexus 8-speed internal gear hub set but I may have had the problem before the change. When I push down relatively hard on the crank arm for example starting off quickly or riding uphill fast, the crank arm slips with a jolt. Quite often the chain comes off soon after. I can't really see anything as I'm always obviously riding at the time, and the noise can't be located, but it is very irritating and happens in all gears.
Would anyone be able to tell me what this could be?