Mystery resistance?

I noticed the other day that my chain ring would appear to either have the teeth cut on it eccentrically, or the ring needs to be adjusted where it bolts onto the crank arm. I store my bike on the wall in the hallway downstairs when at home and the chain is then nearly at eye level. When I turned the cranks I noticed that chain goes tight then slack, then tight again - one cycle per crank revolution - and managed to trace this movement to the front chainring. It still rides ok but I am definately going to sort this out soon.

most fg bikes (if not all) have this to some extent.