I don't have that machine, but there are a few things I would try before taking it in. Change bobbins. Change the type of thread you are using in your bobbin and needle (i.e if you are using dual duty then try gutternans or if you are using guttermans then try another type). Change your needle. Try the machine on a different type of material. Try cleaning out your tension disc...I've heard people use unwaxed dental floss to clean out their top tensions...I have never done so, but it is worth a try.
I know it sounds crazy but sometimes just changing all of that makes a difference.
laughing....I just read the previous post about using the unwaxed dental floss.