I found that the thread needed to be held up in the air by the bar situated to the right of the thread holder. The higher it went the better the tension.
I also cleaned it every 8 hours- don't know if this really helped it, but it seemed to make it better.
Also, watch the speed you sew at- too fast and it makes yuckies on the bottom. (can you tell I've had this experience?)
Barb