follow the vac line from the Waste gate actuator, what is that connected to???? that is what is controlling your boost right Its either gonna be hooked up to a manual boost controller, factory boost control solenoid, or an aftermarket boost control solenoid

 

My car did this to me as well and it was my TB seals. How did you do your boost leak test?

 

Car starts missing or it like shuts off? If it is the boost leaks or ignition problem than you normally feel it like when you just starting to go into boost but if this is happening only at high rpm I think you are hitting the fuel cut. How about you turn the boost down and see what happens. But for sure check for boost leaks and you can change the plugs it would not hurt. I dont think they cost too much.

 

When it was my TB seals the problem dint happen till around 5500 rpm's for me. I was well under boost when mine would cut off.