OK, range toy. In that case I think it's as stated before. It is a heat soak in the action/chamber/barrel area. The pressure/torque surface is moving.
Have you pulled the action from the stock and looked down low to see if the receiver ring is cracked anywhere?
I have a "Cracked Eddy" that I worked over for the heck of it. I have a barrel vise and an action wrench so I unscrewed the barrel, drilled, V'd and welded the crack w/o getting into the thread roots too badly. Dremmeled them out then reassembled. JA 2-groove barrel on bubba sporter ...
But, because I knew I had tweaked the receiver "ring" with the welding heat, I could not trust the torque to hold and I had definitely moved the mating face. So after starting the threads back in, I slathered JB Weld on about the bottom 1/4" of barrel threads and wrenched back to the index marks.
Cleaned it all off with alcohol and let it set two days. Re-installed in stock and went to shooting a week later. 2~3" groups at 100 yds which was much better than "as cracked". No real POI shift, just that spread. Put a box through it in one range session ( Rem Cor-Lokt 150's ). Held basic pattern.
I'm sure the JB locked that barrel in for ever (lotta heat maybe could get it loose...). JB is good to 500* or so, so it ain't moving and good enough for a Hog gun
I'm not suggesting you do this. If your receiver is solid and has no cracks, I'd take it to a competent Smith and have them face the receiver and re-install the barrel. If they say the barrel is shot out, that's your answer. If they say the barrel is good to go, you should end up with a nice shooting rifle