Fred, the OP plainly states that the rifle had been pressure-bedded during the search for accuracy.
It's been interesting to come back to this thread. None of the last three possible problems suggested display the symptoms described in the original post. I know this because I've owned rifles with all three problems.
On the other hand, barrels with some internal stress often do shoot better and to a different place when they heat up during firing. Such stress is usually caused either during manufacture (especially with button-rifled barrels) or when a barrel is straightened after rifling.
Cryo won't cure it, but heat-treating might. But why bother? Instead replace the barrel that's been made right.
