My bike setup produces two tight spots and two loose spots in the chain in each cycle of crank rotation.

In addition, one of the tight spots gets tighter, and its corresponding loose spot gets looser, about every three cycles of crank rotation.

This happens because of an out-of-round condition, or lack of circular perfection in the sprocket, the chainring, and the crank star.

Any thing less than a perfectly aligned frame will exaggerate the effects of all these imperfect circles, and the sum of the imperfections will add up periodically, usually in relation to the ratio between the chain ring and the sprocket.

Further, some very light chain rings will actually flex under load, thus losing even more of their roundness.

One pays premium prices for premium cranks, chain rings and sprockets for their relative roundness compared to cheaper components.

Additionally, the Sugino Zen series of chain rings have extra stiffness built into them, with little weight penalty.

http://www.businesscycles.com/tring-sug.htm