Uniform Striation/Rippling in Y-Axis Only?

Hi guys, I have a Printrbot Play. When printing larger parts, I get strange, severe, patterned vertical ripples (parallel to the build platform) in the y axis only (That is, on surfaces perpendicular to the motion of the y axis). Surfaces on the x axis (in-line with the y motion) have zero artifacting and come out completely smooth.
This effect is greater the larger the part's dimensions in the y-axis are (so the further the travel distance from YMin to YMax of the part).
http://i68.tinypic.com/2qmqjk8.jpg
http://i68.tinypic.com/2le6kuc.jpg
There is a very obvious pattern here. But it's a strange one; almost looks like curly brackets.
The y axis belt is tight.
What could possibly be causing this?
Thanks for any and all insight.