The purple one you have, it is the "BigFoot" FMQ foot? I have one of those and it skips stitches on my 301. If I change nothing other than the foot - ANY other FMQ foot I've tried - works on the 301. I suspect that it's a little too tight a foot and hangs up every once in a while and does what you're talking about - if it hovers, it skips. I've considered modifying the foot, but I really don't have that energy at the moment.
