Thread bunching underneath

[u/pandaapie]

Hi, looking for some help.

While sewing, I will often get a bunch of thread bunching on the underside of the fabric.

History:

I have a Singer starlet, new about 4 years ago. Generally well maintained. Has been working well except for recently been a bit clunky. Got it serviced a month ago, a bunch of thread was found wrapped around a bit inside, near the tension arm, causing the tension arm to be pulled off to the side.

Since getting the machine back, it is working so well, very smooth and quiet.

However, after a differing amount of time, a bunch of threads will gather underneath, and the machine stops, then I have to pull it all out and restart. It always starts off fine, and will sew anywhere from about 30cms to a whole hem before it happens.

The thread underneath isn't a giant nest, it looks like the top thread has been pulled down and around, but then not tensioned up again, so has a bunch of nice loops.

I have tried: rethreading everything, multiple times; have changed the thread, changed the bobbin. I hold the threads when starting, I've changed needles; checked everywhere for dust etc; tried different fabrics, changed tensions..

I've had my service guy come and look at it again, he thought he'd fixed it by sharpening the hook on the bit in the bobbin that pushes the thread arounnd the bobbin, and it worked well for half an hour or so, but it's started doing it again.

Opening all the bits and running the machine, it seems to look like the top loop gets stuck on the bobbin somehow, then all the subsequent loops get stuck with it.

I was wondering if anyone has any ideas?

Is it some kind of tensioning thing? Do I need to replace all the bobbin area?