I have a 2002 s80 T6 with 137000 miles. Car runs good. All maintenance is up to date. Timing belt spark plugs, air filter etc. Just recently the car seems to drop off/ loose power right around 3500 to 4000 rpm. Sometimes it will even drop a few hundred rpm once it reaches 3500 -4000 before i let off the thottle!! Once i floor it, it still feels like it's making power up until I get to the point where it backs off. I recently put in a k&n panel filter and replaced the vacuum lines to the bcs with silicon lines. It ran fine for about 2 days and then all of a sudden boom it started acting weird.
I had a similar problem but it didn't just loose power it was like hitting a wall and then it would stutter. It kinda of made you feel as if you were going forward then all of a sudden being thrown back then forward again and then back and so on and so forth. I discovered that the maf was bad so I replaced it and it ran perfect.
One other thing, i have started to notice what sounds like compressor surge. It happens pretty much every time I let off the gas after giving it a fair about of throttle. Not floored but enough to put you in the seat a bit. I.have been reading online and haven't found any answers so any help is greatly appreciated.
Thanks
Casey

 

I had a similar issue with my 2005 S80 T6. Turned out to be a faulty turbo bypass valve. cheap and simple to fix. Good luck.

 

My first suspect would be the electronic throttle body.