I have two vintage 30-30 Marlin 336s (JMs, of course). One was made in 1960 and has micro groove rifling, the other was made in 1953 and it has the Ballard rifling. They both shoot very well using the 150 grn. Speer Hot Core Soft Point Flat Nose bullets; pretty much made for 30-30s. Right now, though, they're kind of hard to find; Speer says they're a specialty bullet with a potential run coming up shortly at the factory. Hope to see them on the shelf again soon.
Anyway, I bought both of these rifles through gunbroker about a year ago. The 1960 made rifle has a Leupold 4X compact on it, and is wonderfully accurate using the Speer bullets I mentioned earlier. I did try the Hornady 160 grn. FTX LEVERevolution bullets when I first got the rifle, and at first they were extremely accurate at 50, 100 and 150 yds. Then, and for reasons that still perplex me, those bullets went south and started key-holing from that rifle; no accuracy at all, none. I never had anything like that happen to me with any rifle I've ever owned before, and until that session, I'd never seen what a key-holed shot on paper looked like. Yup, perfect side-ways punched bullet holes through the target board. But those Speer 150 grainers brought back the accuracy on that rifle to where I'm completely satisfied with it's current performance. The last 100 yd. group I shot with that rifle/scope combo was 5 shots just a tad over an inch, and that was just rested over my day-pack. I'm totally good with that.
My old '53 vintage, Ballard rifling model 336, shoots the same ammo just as accurately, though with open sights my groups aren't quite as good (read nowhere near) at 100 yds...old eyes. But at 50-75 yards that rifle is "dead" even with my old eyes. Lately I've been shooting both those 336s because they are so much fun to shoot, and easy to reload for. I've been using CCI 200 LRs, and 34 grains of Varget for propellant and full length resizing for each reloading, along with a fairly good crimp on the cannelure; same ammo for both rifles. No need to change a thing there.
Maybe, as has been already mentioned, a different bullet is in order to try in your rifle. The literal "life to death to life" on my scoped 336 with a bullet change may be all you need to make you and your 336 happy. Good luck in your quest.