I'm leaning towards the idea of a burr on the bobbin case. Run your finger all over it to see if you can feel anything. You might want to invest in a new bobbin case to see if replacing it solves the problem. It sounds as if your repair person missed something. If was completely serviced last week, it's probably still under warranty. I would take it back (possibly also with a printout of this thread) and have them take another look at the bobbin case, etc.
Edit: I think this is the single hole plate that would fit your machine:
http://www.sewingpartsonline.com/nee...129642-03.aspx
However, I agree that this is not the solution to your problem -- unless you want to change how you do your quarter-inch seam. With the single hole plate, you would need to place guides in front of and behind the feed dogs (instead of next to the feed dogs), because you would not move the needle. For physical guides I like to use strips of adhesive moleskin (available in the foot section of pharmacies).