LTR 450 issues can't figure it out Please Help?

I have 06 LTR 450 and have been trying to figure out this problem im having for 2 years. Here is what it does it will start and idle just fine sounds great no unusual noises then i start giving it gas and around half throttle it starts to cut out spitting and sputtering and if i give it all shes got it will die. It will do this even if it is sitting there in neutral. It has been sitting a while because i had gotten fed up with it and today i started messing with it again so i got it started and again idled just fine and once i got to half throttle started cutting out. But seems like now that it is warmed up it won't even start it tries to start but won't keep running. I have checked the valves for the right clearance and i got them within spec, i have put a new TPS sensor on it, And not any of that has helped And i don't want to take it to the dealer just for them to charge me anywhere from 500-1000 dollars to fix it. So if anyone has heard of this issue please help.