It sounds to me like you have a string that is breaking in and stretching some. This is going to happen, especially with most generic strings. One thing that most shops don't do, however, is put enough twists in a string, which makes matters worse. The string should have about one rotation (twist) for every 2" of string length. This'll help stabilize the string. Then take it out and shoot it without the tubing on for about 50 shots or so and see if things settle down. Then attach the tube. You may get lucky and find you don't even need the tube anymore.

Of course, this assumes that it's the string twisting and not the tube. I've never seen a rubber tube twist yet.