Probably the ring was mated to the arm off centre.
Having elinated the BB, this must be the problem. How i wish i had got better crank arms (spider and chainring bolts etc)! I think this would have been more versitile in terms of adjustments (chainrings, chainline and tension). Morale: get a circular chainring setup. This chainring setup is just too low spec for use without any chain tensioning device (derailler etc) that could even out the bumps. Yes, before anone says so, I know I can't have a chain tensioner, and don't want one anyway! For a £100 ($194.217) experiment it has done its job. I am a convert to the fixed gear. When I have mashed the cogs and skid-stripped the tires to the ground I will re-build better, newer smarter!