The slight shudder in power is typical of a wastegate flutter common in near-capacity internally gated turbos. In other words, it's a power restriction when you feel it but not one that's going to hurt things. Also, your description of "70% throttle" screams wastegate to me, the car likes to keep exhaust flowing to the turbo but bypass SOME of it around, leading to dual streams and fluttering in between is your internally gated rather pissy wastegate wondering where all this extra exhaust gas is coming from when your engine is only 2.5L and why your right foot can't make up it's mind...
I don't really have any more info for you than that unfortunately. Good luck!
2015 STi: Still Stock
04 STi, 04 WRX: SOLD