I've had a pre-'64 M70 Fwt. .30-06 for some time. Rifle is all original, no bedding modifications, etc. I can get 3-shot, 100 yards groups of an inch, sometimes a little less, as long as the barrel isn't hot. Recently, using the most accurate load I've found for this rifle, I fired a couple of groups that were around 5". I immediately figured the scope had gone bad and switched it. Group size with the different scope was identical. I cut a 1/4" strip about 1 1/2" long from a laminated card and forced it with some difficulty between the end of the forearm and the underside of the barrel - a very tight fit. The rifle immediately shot better than it ever has in the past, but I don't know how permanent my fix is.
While the pressure under the forearm seemed to fix the issue, the sudden change of your most accurate load opening up to 5" implies something changed to cause it if I'm reading your post correctly. Perhaps an action screw loosened, or maybe something was binding in the stock in either the barrel channel or action area that was alleviated by your adding that pressure strip. I'd be inclined to remove that front strip and check the action screws or other potential issues and see if that fixes the problem.
