wavy knit edges

[temptinglies.livejournal.com]

whenever i sew with knits, i get this effect--i don't even know what to call it--on the cross-grain hems:

do you see how the knit kind of spreads out and becomes wavy at the edges? this never happened to me when i resized t-shirts. i've messed with the tension, tried different kinds of stitches. what am i doing wrong???

thanks in advance for any advice you can give me!

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