I had the exact same rifle. was accurate with light bullets e.g. 45 Gr. and heavy charges.
Mine was only so-so with heavier bullets - Slow twist I guess.
I also have a MKII, a stainless model built in 2000. Like yours, it does best with light bullets. In fact it does so well with 40g V-MAX and BT I've quit trying anything else in it.