My chain slackens, rhythmically, while pedaling. While I know that these types of questions draw the ire of the gallery, I searched for this specific problem and came up short.
While pedaling my chain slacken on a specific area of the chain, and I've got that thing "tight". When the chain hit's the tight section it's pretty damn tight.
I'm thinking that it could be a chain-ring issue, or worse, a crank problem.