Here is an excellent article that may answer your questions

Scope shift & barrel droop...two common problems

By the way, I have a RWS Diana Model 38 that I experienced similar problems with. The cure was buying a RWS scope mount that was adjustable for barrel droop, and also installing a scope block. My RWS is a magnum air rifle capable of pushing the pellet at 1000 fps. Also, have you thought of installing one of the "suppressors" at the muzzle? They don't really do anything to reduce the sound, but they give you something to grab on to when cocking, and they add a bit of weight at the muzzle.

Best of luck,

Dave