Is there any way that you can post closeup pictures of the stitching on the top and bottom of your fabric?
Usually large loops of thread on the back are the needle thread - not enough tension on the needle thread to balance the tension on the bobbin thread.
But, you say this is the thread from the bobbin with big loops, and if that's actually the case, it probably means that there is too much tension on the bobbin thread. This can be caused by overwinding the bobbin past that ring marked on the bobbin or by the bobbin thread being caught in something, or by "crud" packed into the spring on the bobbin case.
Since the machine has worked well before, you just have to keep trying different things. The first thing to check is that the loopy thread is actually the bobbin thread and not the needle thread. (I use two different colors of thread when I'm troubleshooting a machine to make it easier to tell the difference.) If it's the needle thread, the thread is probably not seating properly between the tension disks, or somehow the tension has been dialed down to too low a number. Turn it up a few numbers and see if that helps.