In most cases, this is caused by loose threads and fluff being in the upper tension unit rather than a problem with the bobbin. There is a screw on the bobbin case that losens and tightens the resistence. However, it can easily come out and then you have a mess.
Try this to check the bobbin thread tension -
Take the bobbin case holder out of the machine. Use a bobbin that is about half full and use standard 50 wt sewing thread.
Take your needle plate (the metal plate over the feed dogs that has markings showing stitch width under the presser foot).
Thread the bobbin thread in the bobbin case, holding the case so that the thread is through the tension spring as though it is in the machine, tied the end of the tread to the needle plate and have it hang below the the bobbin case.
If the needle plate immediately drops to the floor, unwinding the bobbin, the bobbin tension is too loose. If you jiggle the bobbin case slightly up and down and the needle plate moves slightly down, it is just right. If it does not move at all it is too tight. The resistence should be 14g which is the weight of the needle plate.
The upper tension unit, look straight down from the tension unit and you should see some metal disks which the upper thread rides through when threaded. Take your small cleaning brush and brush out any loose threads/fluff there. Look down the left side of the tension unit to make sure there is none hiding.
When you tighten the upper tension unit by pushing the tension buttons on the screen, do you hear the tension disks tightening? If not, it may be the pc board or the tension unit itself that will have to be replaced.
Otherwise, you might want to take it to an authorized husqvarna viking dealer for service. You can locate a dealer near you by visiting the husqvarna viking website and clicking on the Dealer link on the left-hand side of the page - http://www.husqvarnaviking.com