Blong,
Unless the muzzle was belled out during manufacturing you should really not loose any meaningful barrel length, 0.020" or so.
Soudns liek to me that you have a problem with either the bedding or a stress in the barrel that pokes its ugly little head out when the barrel warms up.
Did the bedding help consistancy any from before to after as far as groups and that third shot opening up?
Is this a sporter rifle, I assume it is with the 22" barrel length, to be honest, if you are getting consistant 1/2 to 3/4" groups from a factory sporter I would not look a gift horse in the mouth so to speak.
Sadly this is better then most will do. Recutting the crown will only make sonsistancy better if done correctly so you really have nothing to loose but it sounds like to me when the barrel starts warming up it is either expanding and touching the barrel channel or there is a stress in the barrel that is flexing the barrel as it warms up opening up that group.
Does it get worse with the 4th, 5th and 6th shot???
Good Shooting!!
Kirby Allen(50)