Diana Consistency problems Diana 340 N-Tec

I have a love hate relationship with my Diana 340 N-Tec. It's beautiful, well made and inconsistently accurate. I mounted a scope With the angled shim type adapter to compensate for the known barrel droop. I have tried many pellets. It is a 22 cal. I have great sessions and sessions when I can't hit much. Any advice for me? Is there some maintenance item I'm missing? What is your experience with this gun?
 
Clean the barrel, especially if it’s never been done. Check that screws are tight. What pellets are you using and have you tried others? Read through the Tips on Shooting a Springer thread at top of this section. All of the preceding is a good start.
Wow that's a great thread. Thanks, I'll start with the fundamentals