I had to have my PC6500 serviced for the same thing. Threads kept breaking and I ignored it, then thread started to wind around a bearing in the tension/threading assembly. I had to have a new assembly put in. I asked what caused it - thread, after so long starts to wear a groove in the bearing since it is hard rubber or plastic. Once that happens...
Hopefully it just needs a good cleaning. I must say, though, that since I had the machine in for repair and a spa treatment it has never run so quiet.

I take my machine in for a good cleaning every year. I can hear when it needs oiling - the needle gets changed with every project.

That's happened to me, too.

I'll be watching this thread - my Christmas present was a Brother SE-400. So far, it works great and I love it.

It's interesting that you can hear that the machine has a problem. Can you isolate where the choppiness is coming from? That might help with trouble shooting the problem.

Recently my resident handyman (DH) cleaned my HQ16 after 2 years of medium use. It was spotless except for a strand of thread caught in one of the gears. It hadn't caused problems before the cleaning, but removing it means that it won't cause problems after.

Mitch|'s Mom mentions something similar on her PC6500.

Good luck!