If the thread is breaking and you have long stitches, you need to work on your hand speed verses machine speed. If you can set the speed down on your machine, put it so if you press your foot pedal down completely it will only go a comfortable FMQ speed. If you are not using Machinger or similar gloves to help move the quilt, get some as they really help.
Accept that you will never get as perfect a stitch length as when you are regular sewing. When FMQ the stitch length will vary.