your thread fraying/breaking problem is more than likely because of an incorrect size/type of needle.
many machine quilters use polyester threads- aurafil is another nice quilting thread- but really you can use any thread you want- just make sure you use the correct needle for the thread choice.
my viking is quite 'picky' when it comes to threads---it has taken me years to figure out what i can and can not use with it-
if you are unsure about the right needle with the thread choice- visit 'Superior Threads' they have free charts you can print off- explaining the different types/weights of threads- their uses & what needles should be used with what thread- very good resource to have.