The bolts that tighten up the worm block have this washer which should keep the worm block from moving when you tighten it. Check and make sure that you have these washers. In the pictures section under\adjustment on the first page are some pictures which I think show this washer. It's not that hard to adjust the worm so that it runs smoothly and doesn't have a lot of backlash although it helps to have a second pair of hands!
Good luck
