I too have developed tension issues on my 500. It was sewing very well and then all of a sudden, the upper tension was practically none existent. If I continued sewing, it may or may not correct itself. Sometimes, all I needed to do was re-thread it, but then after a few minutes I would have the same problem. I finally took the tension apart and cleaned it. I could not see any dirt/dust that may have caused the issue. Spent the next 30 minutes sewing OK. We will see, now that it has 'rested' for a day. LOL
For the record, I am using Gutermann thread.