If your chainring is pretty far out of true, you can take it to your lbs, and they can true it up for you. I had this done to my bike, and it helped the drivetrain run smoother. So if you're still having problems after centering your chainring, you might want to give that a try.
is the chainring out of 'true' or is it out of "round'? I think like wheel true would refer to side to side movement. round would be diameter. tight and loose spots might be a sign of the ring og cog not being round is your axle bent?