That's exactly what I was going to suggest! I moved my frame some time ago and for months could not get a straight sew across. I finally got impatient enough to do something about it! Turned out the frame was not "square" and that created a curve in the row while quilting. I have also experienced the exact thing you spoke of.
I have plastic rails on my machine, too. After a short period of jumpy quilting, DH and I took the machine and carriage off the frame so we could examine the rails. the rails had literally crumbled in place, thus causing the jumpy stitching!
Hope your problem resolves quickly.