Perhaps a few drops (or a vertical line) of the silicone type lubricant which I believe is called Sewer's Aid on your thread might help. It may give a touch of lubrication to your upper tension and ease the passage of the thread. I love using a 90 topstitch needle for most of my sewing. I do not have LA or midarm so do not speak from experience on one...only garment sewing and piecing and FMQ on my DM. Good Luck, I truly understand your frustration...I'll say a prayer for your peace of mind.

Have you tried a top stitch needle? I use a 16/100 or a 14/90. I use them and have no problems with machine/thread misbehaving.
Sharon

I had the same problem on my Pfaff Grand Quilter. I was told to try a topstitch needle as it has a bigger eye.

I have a 1600 P and I teach MQ 101 on it and have quilted over 100 quilts on it, and the only threads I seem to have problems with are sulky and connecting threads. I almost always use king tut, coats and Clark quilting thread, and mettler. I use a Organ 90/14 most of the time. If the thread is old it will break. I hold the thread up to the light and some have small burrs every so often and when you pull slightly on each end, that is where they break. Also, make sure you loosen your pressure foot tension, as that can also cause problems. I really love my machine, and I have a lot of different brands. It is a work horse. So sorry for your issues. Also, overlock thread is a thread that is very thin, that is why they use it with 3 or more cones with it on a serger. Not recommended, especially for a machine that runs at such high speeds. Also, if you are free motion quilting , don't quilt faster then your machine is sewing. Adjust your speed control.

courtney where are you in North Dakota? I am over 50 miles nw of Minot.

This is a good point, I hadn't even considered it. Your thread might be dry. Sharon Schamber dunks her cones in mineral oil. I have a couple of friends with longarms who have tried this and report that yes, it does work and no, it doesn't leave oil spots on your quilt (as long as you use the correct oil, pharmaceutical grade mineral oil).

Courtney, I have the evil twin of your machine :( It is currently at the store where I purchased it from for the third time. I saw your problems and they exactly match what I experience with my machine, the thread breaking, problems going side to side. I also have had problems with skipped stitches. These problems mainly occurred with Superior SoFine 40wt thread. I would happily quilt dense background stippling with bottom line in the top and bottom and when I changed to the heavier weight thread for a different part of the quilt, everything went downhill. Luckily I have another machine (Janome 6600P) it worked beautifully with all the threads and I was able to finish the quilts in time for Christmas. I tried everything that I could think of - lots of different needles, tension changes in the top and bobbin, different weight thread in the bobbin... The repair man told me to take my threads in and he would test them all. I pick the machine up tomorrow and will meet with the repair man when I collect it so he can go over everything with me. I will let you know what feedback I get. Hopefully it will help you too.