Thread is shredding
 

I have a Janome 6600 and have previously had very good luck with fmq using different types/brands of thread. Today, I was working on a large lap sized quilt, doing some basic stippling in 8" blocks when the top thread would shred, right before the last two thread guides before the needle. This happened several times, each time before I could get one block done. I was using cotton thread in the bobbin, and a lemon yellow poly thread on top (the yellow to show contrast). Each time I would rethread, cut off the bad section, and start again. I even went as far as switching to a different spool and got the same results. When I went back to the cotton thread, I had no issues at all. I just wanted to use the poly for the color. I have some cotton thread in the right color on order, but it could be a while before it gets here. Does anyone have any suggestions on what I need to do? I've used poly on straight stitching with no problems, even mixing threads. This seems to be just a fmq issue.

You mentioned that when you went back to cotton, all was good.
So how about trying poly in both the top and bottom?

Sometimes mismatching threads just doesn't work.

My Janome does not play well with poly so it is cotton thread for everything.

Try threading the take up lever backwards. Instead of from right to left, go left to right. About 80% of the time when I have a thread problem on my 6600 threading the take up lever backwards solves it. The other thing is make sure it's not catching and hanging up for a little bit any where on the thread path.

I say change to a top stitching needle if you're not already using one. The eyes on those are slightly larger than a regular needle and have a channel for the thread to move up and down through.

Ditto the topstitch needle suggestion.

Try the topstitch needle; if that doesn't help, give in to her...she knows what she wants for lunch...;)

change needles!

I concur - change your needle to a larger eye and that should solve the problem I use 90/14 mostly.