There's so many ways to tackle this!
I tried all of the above as you have with no difference. In the end I stripped the rifle and removed the mainspring. Then I could slowly cock it whilst observing what was happening. Guess what- it was obvious. On my rifle the articulated link wasn't working and was pressed hard against the cylinder- that's where the galling was occurring.
All I did was glue a little bit of bicycle inner tube rubber (very useful stuff btw ) just adjacent to the start of the cocking slot. This allowed the link to articulate. Job done- no problems ever since.- Oh. the spin off is it makes cocking a tad quieter!