I had a very similar problen last year with a Janome. Nothing I tried eliminated random skipped stitches, and they were more frequent where the seams met. I finally packed it up until I had time to get it to the dealer. The repair technician had to make some adjustments that were covered under warranty, despite it being well out of warranty. Now it's sewing beautifully. Don't bang your head against that wall too long... it may be the machine. Also when sewing through thick layers I increase my presser foot pressure.