It sounds like to me your doing everything right. Whether changing to pellets over powder will make a big difference I am not sure. I guess it can't hurt to try.

I am wondering if by the third shot if the barrel has not heated up a lot more then the first three shots. An over heated barrel can play games on the plastic wad of the powerbelts or sabots. Try letting your rifle cool down longer. Some people even go the extreme of taking cold towels and wrapping their barrels between shots. I noticed a big difference in accuracy from shooting on a hot day and a cold day.

Something I should ask is, are you taking the plastic base off the powerbelt and painting the lead spike of the conical with some grease, bore butter, oil, something... This will help the powerbelts shed the base faster and give you better long range accuracy.

I am not saying you do this (although I do) but some people after extended shooting, will lower their concentration level or raise it higher then normal. They start straining the shot or just get shooter's fatigue and relax too much. They then throw one and that brings them back to their normal levels of concentration. I overcome this by taking more time then usual in swabbing, and checking the target, the way my bench is positioned, relaxing, anything to slow down....

The best thing to do is stay consistant in your loading and shooting procedures... You have a great rifle there, and it will probably shoot better then you can hang on to it. I have a T/C Black Diamond XR that is an outstanding shooter. I have worked up some really good loads with the rifle and am looking forward to getting the new scope any second now.. so I can shoot some more...

Good luck with your shooting