Thank you all for your suggestions. I took my machine in for servicing, and while it's behaving a little better, I am still getting the intermittent skipped stitches and broken threads, even after switching to a 90/14 top stitch needle, rewinding bobbins, trying different thread... The ruler work is going a little better, but I'm still searching for the sweet spot with the foot... I tried quilting a table runner, and found a couple of places where the seams weren't nested properly, and the foot wouldn't even go over them... need to adjust it up a bit...
I wish I had purchased the convertible foot and ruler foot, but the 3 dealers I called weren't knowledgeable enough to recommend it. That frustrates me a little. I'm off to do more practicing.. and troubleshooting...