I'm just gonna toss this out there, it may be something to consider.
We have that same machine, I have used it for vinyl and it did a good job with it but I had to change out the foot.
Maybe consider changing to an open toe foot. It is wider and you can see your work better, but the part that may help is the fact it has a 'tunnel' on the bottom side, so when you drop the foot it puts the proper pressure on the material so it contacts the feed dawg better..
I suspect what is happening is the stitch is so thick with that heavy thread that the foot doesn't put enough down pressure to make the material feed like it should because there is less pressure on the feed dawgs. The foot is actually being held up some due to the thickness of the stitch.
Do you find yourself having to tug on the material a little bit to make it pass along smoothly?
IF you do this I would go with a metal foot as opposed to plastic just because of the bulk that you are working with. The metal foot is a bit more in cost, but it won't snap under heavy use, IMO.
***I can't say for sure if that's your problem, I do know it helped with ours.
***
Here is an example. Just need the correct foot, snap on, screw etc.
https://www.walmart.com/ip/Singer-O...bucket_id=irsbucketdefault&findingMethod=p13n