1. Check the derailleur position over the cassette cog it is supposed to be on. The upper jockey wheel should be directly over the cog.

2. Check your chain for wear. Check you chainrings for wear, "shark finning" and bent teeth. Installed a new chain recently? A new chain on worn components will skip under even slight load. If everything is just really worn out, time for new: chainrings/cassette/chain. Yes, all at the same time.

3. Depending on the age of the rear wheel, could be a dying freewheel, or damaged freehub. This would be a sharp "klank" kind of sound, with not as much movement as actual chain skipping.

I can't think of anything at the crank itself that might cause this.