Sorry, but I'm probably not the person to ask. I work on the industrial side of sewing, and my home machine is this thing.

If you're using heavy thread, you definitely need a bigger needle. Maybe a "denim needle"? The thread should slide easily through the needle eye with plenty of space. Also, turn up the tension on the top side, and make sure it's threaded correctly through the tension plates. That "jam" you're getting is probably because the take-up lever isn't pulling the thread back up through the needle (the loop is jamming up the next stitch).

Like I said, advice is hard over the internet with sewing machines. Go back to what worked great, find the variables (thread? material?), go from there.