Dan, I previously owned that telescope, and implemented some of those mods (shortening the tube and upgrading the secondary). I did not see the diffraction pattern artefacts you are observing. eg
http://www.astrobin.com/full/153530/0/. As such I suspect that Dennis may be right in that the long screws are to blame.
edit: collimation may also be out. I previously used catseye collimation gear for that scope.