Collimate your primary then fix it using the locking screws. Check your collimation again as the locking screws themselves may vary it. Now move your scope up and down. Does the collimation still change after you lock up the mirror?
I had a similar problem with my 12" Lightbridge. The primary mirror springs were so puny the collimation screws just didn't work properly. In fact, I used the locking screws as the primary collimators. The problem was solved by replacing the springs with heavier ones.
Refer to these recent threads ...
http://www.iceinspace.com.au/forum/s...ad.php?t=59148
http://www.iceinspace.com.au/forum/s...ad.php?t=58731
Regards, Rob