I too have had a number of problems with my Janome 6600p, although usually (and I mean usually, not always) have not had issues with the tension.
I've had better luck using Guterman's poly thread than Mettlar's cotton -- but really couldn't believe that any machine would prefer poly to cotton (and I prefer the look of cotton for topstitching). Was amazed that others are saying the same thing...thought for sure it was just me (operator error).
I suspect some of my problems might be due to timing but I've had the machine serviced once, to the tune of $100 (and the machine is just over a year old) and dread spending that amount again. My most common problems are thread shredding and skipping stitches. I've cleaned the machine over and over, changed the bobbin case (red/blue), the bobbin, the needle plate, the needle, and the feet.
BTW I have two FMQ feet (the clear one that comes with the machine and a metal circle one that is sold separately) and learned that when using the circle one, which has a spring/height adjustment, you DO have to adjust the height for it to work properly. The clear one does not have any adjustment.