This Youtube video on setting longarm tension might be helpful:
http://www.youtube.com/watch?v=Q1mRh...664A7&index=13
Sounds like your tension does not remain stable. I like the idea of replacing the tension assembly spring. Seems to me a spring can become weak, and this may make your machine unable to handle slight changes in the quilt -- such as change in the quilt tension towards the end.
Also, how tight are you rolling your quilt sandwich? You want it loose enough so you can grab a fingertip that is pushed up underneath the quilt.