I've had the 6600 and currently have the 7700. One suggestion I learned from the 6600 Yahoo group (really helpful, by the way) is sometimes threading the take up level from left to right instead of right to left makes a difference. Also, it can help to skip the last threading point just before the needle.
With the convertible quilting foot, because it's not a hopping foot it's important to get it in the proper position. The little screw adjustment on the foot will raise and lower the foot in tiny increments. It should be resting on the fabric, but not too tightly. You might even refer to it as skimming the fabric.
Hope this helps!