Originally Posted by Ramblinrev
Tension is a tricky thing. It depends on a lot of things.
Do _not_ adjust the bobbin tension unless you know exactly what you are doing and why you are doing it. For more information on bobbin tension read my post in my signature. In the vast majority of cases, the problem can be addressed with top tension adjustments.
Here's another respected guide to learning and knowing what you are doing. There's more here than appears in the top-rated youtube video.