Not clear from your post if the speed control issue only happens with cuben or across the board. If the latter, you may need a better speed control pedal. You could pick one up at a sew+vac shop and wire and tape it into the the line that plugs into your machine. Also, the machine will operate more smoothly if kept clean and lubricated inside. Hope you don't need a new motor. Do not use a full-size presser foot for straight stitching - the thin two-pronged ones are good, and work much better for me on silnyon.
I have not sewn with cuben yet, but decided some time ago only to sew together edges that have already been hemmed by folding over and bonding. Too much on the forums about the stitch holes being pulled open by the thread, as well as pictures of puckered seams. Haven't decided yet whether to bond with one of the tapes or GE silicone glue. Will need to see first how the tape adhesive affects sewing, and how well the silicone glue holds. Would not go to the trouble of using a urethane glue unless there would be no stitching at all.