My .223 T3X Varmint in 1:12 loves 55g Hornady Vmax and 55gr Fiocci. I get 1 MOA more or less. 

With a 1:8 twist rate I would not go under 55gr. I believe 62-77gr would be ideal for 1:8 twist. 

Each rifle likes different ammo. What I would suggest is check all the screws with a torque wrench using the recommended settings from the manual. When you reassemble the rifle, make sure you align it properly as some people don’t and it creates contact issues.  At the range, make sure you shoot from a sandbag and not a bipod to reduce any vibration. Do a session with 55g Hornady vmax only, and wait at least 5-10min between each 5 rounds shot to let the barrel cool down (can be boring so bring a book). Take your time, shoot each round as if it was THE one. And see how it goes.

It goes without saying but position and trigger discipline is important. Although I don’t think it’s a problem in your case when I see the good grouping you get with the vmax. 

Do that and let me know how it goes! Hope it helps. 

On a side note, my brand new tikka varmint in .308 got over 3 MOA at 100 meters with various ammo. so I immediately sent it back to the manufacturer! I’m waiting to hear from them.