Richard, chronograph your rifle, you will probably find that it is punching out under 2000 FPS.
You need to increase the velocity to above 2150fps to stabilise, the Mk.VI round was designed to punch at around 2250 and the Mk.VII at 2450 fps.
The loss of velocity thru blowby caused by throat erosion can be partially overcome by judicious handloading, working up a specific load tailored to the particular rifle.
Have fun.