Hi Erica,
It's strange that you can knit so many rows before the problem arises.
Quite often, when the carriage catches the needles it is because the sponge bar is worn and needs replacing. It might be worth checking yours - the foam should be at least 1/4 inch above the metal all the way along the bar.
Also make sure the knitting is weighted evenly and move the weights up as the knitting lengthens.
Hope this helps,
Sue
August 2011