jackruff: I would hazard the guess that your bullets are not properly sized or moving fast enough and that tumbling is the result. The twist in your Marlin barrel might require a faster load with a properly sized boolet. Also, for both you and the av-doc, Marlin barrels are both oversized and possibly micro-grooved, which means they need to be slugged and fire lapped.
For this business I strongly suggest you visit Veral Smith's forum, read what he says about fire lapping or barrel lapping and then purchase his lapping kit and instructions. I believe that Veral Smith first developed the barrel lapping/fire lapping process and I do not recommend anyone else unless they follow his process.
My Winchester M94AEs 444s wear Marlin Microgroove barrels and before I could even think about shooting cast slugs accurately I went the Veral Smith route and lemme tellya, it made all the difference. For hog, Elk and me, there is nothing better than when I can spit a .432 300 gn gc fn slug from my 20" Winnie at 2300'/sec and a .433 335 gn gcfn from my 26" tube at better than that. And all this in under one inch at 100 yd from either rifle....
There are others that sell gas-checked slugs for the 44/444 in varying diameters if you do not cast your own. Lapping may also help reduce leading in your SBH barrel too. HTH.