There is some practical info already in the comments above:
Stop cleaning so much.
Float your magbox (my last M77 needed a little dremel attention to get there)
Finger tighten that middle action screw, bear down more on that front screw
Handload some straight ammo, utilizing that long Ruger mag box with your OAL determinations
My last M77 also had a high amount of pressure from the forend contact, so I floated it and bedded it when it started giving me vertical stringing. I'd only do this if the other steps don't work.
