I bought a Janome 7700 in June and I am finally getting used to the quirks of my machine. I had some tension issues with mine in the beginning and I had to fiddle with my top tension a lot.
If by jamming, you mean that the fabric doesn't catch at the feed dogs and it jams under the presser foot, I found that I need to have lots of material under the presser foot and the feed dogs need to covered with material before they will move the fabric. I would suggest using a leader and ender when you are sewing, especially small pieces of fabric that don't cover the feed dogs.
No problems with thread at all. Been using Connecting Threads cotton in it and it works great.
The 7700 is a great machine and has lots of cool features and stitches. Don't give up.
Last edited by IrishgalfromNJ; 12-20-2015 at 05:17 AM.
Reason: to add more info.