I've having similar issues with my .270 Weatherby Mark V. It was shooting boomerang shaped patterns, going from dead center and hitting high and right to left counter clockwise. I sanded the stock so I can get a dollar bill all the way to the receiver. It now will shot 5 shot groups that are about 1/2" wide left to right but the 3rd, 4th and 5th shots climb. I've been told to wait 3 minutes in between shots.I've been told to do a good cleaning with solvent and copper remover. And I've also been told to ditch the lead sled. I guess some people have cracked their stocks due to all the pressure coming back to the grip. Other people have said sandbags are more accurate to shoot off of. I gave mine a good cleaning yesterday and hopefully will get some shots in today. I will see if the cleaning alone helped and if not, will try waiting 3 minutes in between shots and post the results.
After much reading and advice though, I'm reaching the conclusion that it is a magnum hunting rifle and I should probably be satisfied with a 1" 3 shot group.