Chain ring roundness, bottom bracket shell alignment, bottom bracket spindle and cog roundness all effect the tightening and slacking of the chain.

You can throw money at it and buy a rounder chain ring, a rounder cog, a more precise spindle and a more precise frame.

Without spending money, and instead getting the most out of what you have, you can research Sheldon Brown's method of centering the chain ring.

Centering the chain ring will make things better, but it won't cure all of the cyclic tightening and slacking.

And spending money won't get you perfection, either: it will only get you closer to perfection.