Sounds about like what I experience with my daughters Weatherby Vangard. It will put the first 3 rounds into a nice 1" group. As the barrel heats up, the grouping opens exponentially.
So far I have found that the stock was unevenly pressed up against the barrel. I have since free floated it.
Then I noticed that there is a lot of movement in the forestock. I used a Dremel and made grooves in the forestock lengthwise, and crosswise, and opened up the channel some more so I could glass that in. It is much stiffer now, but still same problem.
Next is a new crown...