NYBH,
I have a Summit Python that I use almost ALL the time I also have the same harness setup that you do I dont have any problems with mine. I put my rope about neck high when standing and slide my prussic knot down the tether until i haved enough freedom. I dont see how it could slide past the top stand cable at most BUT i have never had my tether slide and i climb a variety of trees mostly pine. hope you figure it out and make sure you wear one of some kind