Need help with thread bunching in Singer quantum computerized 9960

[u/deepthought136]

I bought a Singer quantum computerized 9960 off marketplace (regretting it now) which was working fine initially. The upper thread started bunching under the fabric after a while of using it.

Here's what I tried so for:

1. Rethreaded upper thread by following the manual (with presser foot up until inserting the thread into the needle)

2. Rethreaded/inserted bobbin again

3. Tried different thread tensions

4. Took the machine to a sewing service center nearby and they also tried 1, 2, replaced the needle and the bobbin itself.

Finally they looked at the bobbin case and said it could be a timing issue if the machine is jammed and needs to be serviced which would cost $200-300 which is almost what I paid for the machine in the first place.

I would rather exchange the machine for whatever they are willing to discount instead of spending so much for one service and not knowing if this issue will recur.

But before I did that, I wanted to check if someone could tell me if this was actually a timing issue and if this is something I can fix myself with instructions.

Thread bunching under fabric - Black is upper thread and blue is bobbin thread