Someone mentioned that they use Maxi Lock thread on their machine. If that is for sergers like I think it is, it's a poor quality for piecing or regular sewing. Serger threads on the big cones can, and usually are, made a little more cheaply than regular sewing threads. Since you usually use three or four threads (if not more) for serging, each thread doesn't need to be made as strong as regular sewing thread.
Also, for those using old threads - test to make sure it's still good. Generally, if you can break it easily with your two hands, it's old and should be thrown away...note I said easily!