Check your tension discs ( if that is something you can do ) It depends greatly on what machine you are using that gives you these results. A dollar bill, folded in half the long way, and passed between the discs will help clean out stuff you couldn't see just by looking at the machine.

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The stitches on the back should be the same on both sides on the back side, yours are not.
If the bobbin thread is to loose, your top thread will put the bobbin thread up to the top, evenly on both sides of the zigzag.
If your top thread is to loose, the bobbin thread will be pulled to the top, evenly on both sides of the zigzag.
If either is to tight then the reverse is true.
There for, no matter what you do the stitches are not even on each of the zigzag on the back side.
The machine is out of adjustment. Once it is corrected you can play with the tension. Until then you are spinning your wheels.
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For detailed information on tension, take some time and go to the website of Superior Threads.
They have on line info as well as a free CD about tension and threads.

The bobbin thread is very thin and has a slight stretch so I think this is my issue. The top thread is 100% cotton and thicker in comparison. Thanks for the hints, I'm going to experiment with threads and see how I go.

Before you go to great lengths on adjusting, make sure your machine is really clean. Lint anywhere on the thread path can really mess up your tension.