Little Roo....I am dealing with the exact same thing right now with my 6600. Literally, I've spent the last 3 nights fudging with this quilt. I've switched threads, switched needles,switched needle sizes, changed tension, changed pressure, used the blue bobbin casing, and cleaned/ recleaned the machine, and rethreaded it a million times. I am very experienced at FMQ so I know what I'm doing with speed, etc..
I had a basic little 4623 janome with a small throat and got beautiful FMQ stitches, no messing around. This machine is so tempermental. I love the larger harp but there are ( many ) times I've regretted the money I spent on it.I'm gonna try to raise my feed dogs too.