This is just grabbing at straws,but steel does shrink when cold.I don't think your firing pin would shrink enough to matter length wise,but maybe it warps a little when it contracts.It may be enough to cause a little misalingment in the bore of the bolt.Keep in mind that the pin for the bolt head is drilled for the firing pin to run thru .I don't know if you bought it new,or used,but maybe the bolt head pin has been removed,and reinstalled a little off.I would pull the firing pin,and roll it around and make sure it's straight,then I would put it in the freezer for a spell,and roll it again and see if it is still straight.Like I said grabbing at straws..I would also take a tight fitting brush,and clean the bore of the bolt just like you would a barrel.That big headed pin that goes thru the cocking piece side of the firing pin is susposed to turn freely .I would also check it too.Does the bolt face have any gas scars?..I know all of what I said is a longshot,but I hope it helps.