It seems strange that prior, it grouped fine and now it suddenly will not group at all. I think the 150 grains is too much powder for the powerbelt. I never could get any accuracy out of them past 100 grains.

I am guessing you were shooting good groups at 100 yards in the past. So check your scope mounts first. Make sure you are shooting on a clean barrel. And have someone else shoot the rifle and see what their results are.