If you're talking about the "quality" measurement jumping to 90 and back to zero, you're simply passing up the satellite. Unless the dish is sitting perfectly still when that happens, then something isn't right...
But if you're passing it up, remember you must restrict yourself to very small movements of the dish. By "very small movements" I mean even a slight flex in one direction, without the bolts even being loose, is enough to knock the signal down. To fine tune my own dish, I started with all the bolts very tight, and flexed the dish around until I found the direction of adjustment it liked best. Then turn the proper bolt(s) a couple rotations to loosen, just enough to let you gently adjust it a little, then re-tighten.