It is probably just the "tight" spot in the revolution of the cranks. Most chainrings are not perfectly round which is not a problem in a der based drivetrain because the der makes up for the changes in chain tension. On a fixie it has no where to go so it gets really tight in a certain spot when pedaled, the chain often vibrates up and down rapidly during the tight spots and then droops a little on the looser spots. You could move the rear wheel slightly forward and help it some but it might result in the chain being too loose. Another option is to try to center the chainring on the crank. It can be done by loosening the chainring bolts and then pedaling to where the tight spot is and tapping the chain with a crescent wrench. Sheldon Brown explains this better on his site.
Cheers,
-Ryan