Definitely try new ammo. I don't know what's the best. I've shot a little Remington 55gr and some S&B 55 gr and was able to shoot MOA with it and an out of the box rifle (on a rest but hey). I'm sure match grade would be better but even the ordinary brass stuff ought to do better than what you're experiencing.
I hope it's not the brand or your rifle. I still have a Rossi 357 snubbie that's now broken 3 times (factory service each time). It's still in my safe, broken. If I spend the $ to send it the factory AGAIN, I'm selling it to some shop as soon as it gets back. I hope you didn't get a Rossi rifle like my Rossi pistol...
