Sorry, I am not familiar with a Janome 6609P, and am guessing it's a typo, meaning 6600P?
If so, I am quite familiar with the 6600P, as it is what got me hooked on quilting and FMQing.
You mentioned using Schmetz "quilting" needles.
I have never had luck with the actual "quilting" needles, on that machine or my current one.
My go-to needles are Embroidery and Topstitch needles, size 14.
Not saying that is where I end up, but I always start there.
Also, you said you are doing ruler work when the problem happens.
Are you able to do FMQing without any of the same problems, with the same needle/settings?