Just looking at your shelf, it is not supported in the middle/back. I had this problem with the 6 foot "church tables" in quilt classes...it seemed I was the one that was in the middle of the table. My machine jumped all over the place when sewing fast, or fmq. I purchased an expansion rod....sort of like a shower curtain rod, that when depressed you could slide it under the table...release it under where the machine was. It gave great support under the machine and the bouncing stopped. Someone borrowed it from me and never returned it....I have looked for the exact same rod. I think I will check into the dowel rod....it would have to be cut to the standard height of a "church banquet table"...