Tumbling is caused by the projectile losing stability, which is imparted by the rifling in the bore. Loss of stability can be due to many different things: shallow rifling and a loose fitting projectile, a loose sabot/bullet connection, low muzzle velocity/energy, etc.
Your numbers are good (275 gr bullet, 100 gr powder, 1:28 twist). I'm going to suggest your Pyrodex pellets are being crushed and not fully burning, resulting in low muzzle velocity/energy. Or perhaps they are degraded by humidity (Hodgdon says Pyrodex is adversely affected by humidity) during storage. They can also be easily contaminated by the oils in your fingers or residues from swabbing/cleaning the bore.
Honestly, I have little respect for Pyrodex or 777 pellets. I believe they are inconsistent at the very best, and next to worthless at the worst.
Switch to new Pyrodex RS loose powder, or better yet, loose real black powder and see if that helps. And I'd back down to 80 grains for your 50 yard shots, although that has nothing to do with stability, just accuracy.