Too long for comment, but I suspect wastegate linkage or door not closing fully due to distortion from heat (it's really hot around there). You could try and get it hot enough to duplicate the problem, park, and attempt operate the wastegate manually with pliers or gloves... but it's tricky to get to without removing the turbo heatshield.
Also make sure the boost control solenoid is working, because if it isn't you will be limited to raw wastegate (around 7psi I think) with a 0% effective duty cycle. Which will be crappy choppy (bouncy revs) boost in the heart of the boosting range.
And get a boost gauge - easy install and you should have one anyway. That will tell you a lot. I suppose it could be a "boost leak", like a cracked inlet hose or loose intercooler connection, but the "bouncy rev limiter" symptom suggests a turbine leak more than the compressor side.
If it is the actual wastegate door, you will have to drop the downpipe to even get an inspection mirror look. And while it's rebuildable, maybe it's time for a TD05, VF24 or VF30? Because I'm certain you're rich like I am... or was...
Yes, I'm a Scooby Geek, but not the Subaru Geek. (He taught me everything I know... but I found out later he didn't teach me everything he knew...)