Have you tried different ammo for that weapon? Maybe its not that the Whelen isn't an accurate cartridge, but that your particular rifle doesn't like that particular load. I would try someother ammo and see how the firearm shoots. Have you used this ammo to take game before? Also you say that 3 shots will be in 1.5" and the next three will be all over the place. How long are you waiting before shots. It could also be that your rifle does not like a hot barrel and needs time to cool off between shots. The barrel could be getting too hot, and causing groups to go all over the place.

Hope this helps