Well I am backwards from most at fmq because I go reeeaaaly slow. I am not very good at it and haven't been at it for years like some. But then I also don't drop my feed dogs! I found my stitches were actually more consistent and prettier this way. I found when I used the coats and clark regular quilting threads it shredded sometimes when quilting. I had 4 spools of the coats & clark star someone gave me and it performed fabulously when quilting but it is pretty thick (30wt). I've fallen in deep love with the Presencia 60/3 for piecing and the Essential threads for quilting. I agree with others that it is probably the table vibrating. You could set the machine on a real sturdy surface temporarily like your kitchen counter and just run it full speed to see if it vibrates. That should tell you if it's your machine or the table.