Immortal:
I did a write-up that covers all of the causes of the symptoms that you describe, and their solutions, but it's too lengthy to post here.
Take a look at Mike Focke's website for the write-up, here, under the heading "When the Top Edge doesn't Fall into the Guide Channels":
http://mike.focke.googlepages.com/thetop-itsmaintaince%26replacement
Let us know how you make out.
Regards, Maurice.