Chain slipping every second rotation

[Guest]

Every second turn of the crank chain tension slackens and crank jumps, like chain is trying to catch up on the stroke.

[Alex Ramon]

Sounds like you have a stiff link. Watch the chain as it wraps around the rear sprockets. When you find the link that jumps, twist it back and forth with your hands until it loosens. This video might help:
http://bikeride.com/stiff-chain-link/
I hope that works!