Your wastegate is probably stuck and you're hitting boost cut. Take off the heatshield over the turbo and try to wiggly the wastegate actulator arm. It should be pretty stiff. but you should be able to move it by hand. If it moves than probably the boost control valve is dead or you have a massive vacuum leak somewhere. To eliminate the astegate completely you can take a short piece of vacuum hose and connect the nipple on the wastegate to the nipple on the cold side of the turbo and try to run that way (don't forget to plug up the hoses that you took off.
This is what the wastegate looks like: