problem with brother

I have a Brother 600 and Im getting a message of check upper and bobber thread. I check both, machine works for a few stiches and then I get the same message. I cant find anything wrong with either thread. What could the problem be?

there are a few things it can be:
1. check for fuzz in your bobbin case and around the tension spring in the bobbin case - thisis a very common area for causing this issue as the fuzz collects under the tension spring and causes the bobbin tension to fluctuate.
2.take your upper tension knobs apart - watch the order in which the spring and spacer and felt pads come off - clean the rotary tension piece - make sure it is free of any lint or fuzz - put a small drop of oil on the shaft it slides on - reassemble and set the tension - tension for poly thread should be 100-130 for upper thread and 18-25 for lower bobbin tension.
3. check all the pathways for fuzz or lint - anywhere the thread travels - clean with rubbing alcohol to make sure it is free of lint and fuzz and that the thread is traveling a clean path.

Hope this helps.