I have a Janome 6500, for about 13 yrs, and a longarm Innova 26 in. Having a regular machine on the frame would leave a very small opening. Have you considered dropping the feed dogs or if you already did NOT dropping the feed dogs? I have done many quilts on the Janome 6500 and still do small quilts but only as a sit down machine, not on a frame because I felt the opening would be way too small. I found quilting with the feed dogs UP worked better then with them down. Also use a 90/14 top stitch needle,
https://www.walmart.com/ip/Schmetz-T...&wl13=&veh=sem and thread like Aurafil. Also be sure not to outrun the machine or jerk it.
Please feel free to pm me. I find the Janome 6500 a bit of a thread snob and I went through a learning curve with using it for quilting. I used it exclusively for piecing and sit down quilting for nearly 10 yrs. I did many very large quilts on it like 10 ft x 10 ft. Yes, that is feet. They were all done in sit down mode, not on frame, but I think your issues might be more the machine then the frame.