Thanks for posting this. I've occasionally had tension problems with my 503 and once on my 401. It seems as if the tension is too tight (or too loose), and I give up on figuring out what is wrong after threading and rethreading and adjusting the top tension. Then, the next day it's fine. I almost think I imagined it. It's probably happened 2 or 3 times on the 503, which is very similar to the 400 series.
I'm not sure that's the problem I had, as you can't overfill the bobbin on the either of them. By that, I mean it can't fill past the outside edge of the bobbin as it stops filling automatically. I finally adjusted the bobbin filler on the 401, because the it had to be filled too often. I have contemplating adjusting it again to reduce the times I have to fill it.
I'll follow this thread to see if you get any answers.
bkay