I have the PC-420 too and wouldn't trade it for anything. It does sound to me like the feed dogs might be hanging up somewhere. I had a similar problem. Thought I had fixed it and several yards of sewing later took it apart again to clean out lint and put it back in (apparently better that time) and voila like sewing on a new machine again. I don't know what was wrong but it muddled along for quite a while that way. Hope it keeps working great for you. Keep cleaning it and taking that bobbin holder in and out - eventually it will resettle the way it's supposed to. Hugs!