FX Wildcat Hammerspring tensioner problem...

Got my Wildcat a couple of weeks ago and lately the airgun started to open up the groups big time, and after cleaning it showed no improvement whatsoever.
So after some investigating I found that the brass colored hammer spring tensioning/screw plate at the back is very loose. It had unwindened itself all the way(!), so I had only 10 Joules (7 fpe) worth of energy coming out of the gun through the chrony…

I screwed it back in till it could not go further but the pellet velocity was too high, (290m/s and no good hole in hole grouping) so I adjusted it to get the best grouping, which turned out for JSB Heavies (18 grain) 270 m/s and for the regular JSBs (16 grain) 255-260m/s.

Now here’s the problem: the brass setscrew/plate is so loose, that it unwinds itself about 3 to 5 degrees after every shot and subsequent cocking of the hammer. (I put a Sharpie marking stripe on it so I could verify it stays at the same setting I put it, well, unfortunately it does not.)Is there a fix for this? I think locktite is not a practical solution in this instance, as I want to be able to change the setting (which is 2 and a quarter turn when changing from one pellet type to the other.)Who knows what to do and how to fix the loose setscrew/plate

Thanks in advance for your time! (and Ernest if you read this: thank you very much for all your shared expertise in your instructional videos, very informative and extremely helpful!)

PS. When I first got the airgun the link that connects the 2 arms of the cocking arms fell out into the gravel while on the shooting range outside and was nowhere to be found anymore. So I had it replaced by the vendor. It would seem that the link should have been expanded at the ends (by hammering it) so as to prevent it from falling out. Clearly this was not the case. So maybe they forgot more things to do on my airgun? (like something that fixates the hammersetscrew?)