I don't own one but I sell them - I'd start by completely rethreading the machine, in case you haven't already done that. Since it seems to only the a thread/tension problem, remember that, usually, if the loose thread is on the bottom, it's top tension, and if loose on the top, it's the bottom tension. The Vikings are not designed in a way that you can adjust bobbin tension but sometimes, tho not often, the thread can jump out of the bobbin tension mechanism - if you keep having problems in the bobbin tension, you need to take it in to have it looked at. Depending on when you purchased it, it could be a warranty issue.
If the top tension is off continually, use some of the texturized dental floss (made for dentures, mainly) and run it thru the tension discs to make sure all the lint is out then rethread again.
Other than thread tension - make sure you've completely dropped the feed dogs since sometimes people have not completely slid the switch over.