It seems quite common that shots in a group will start to wander as a barrel heats up.
I have one rifle that does it - not horribly, but they will start to string vertically up to about an inch and a half if I shoot more than about 3 in a row. If I let it cool for 5 mins, the next shot is always perfect again. Neither of my other centrefires does it.
I've come across a confusing array of explanations for why it happens. I'd be curious to see what people's opinions are here. I should add that I'm not looking to correct it, since I've never taken more than 2 shots in a row when actually stalking. I'm just curious - especially since this is a Heym, so comes glass bedded from the factory.