Got my first WRX and it has problems. Help!

[harward2420]

So, today I bought my first WRX. It's 2005, 47k miles and it is completely stock. The problem is, when I accelerate as soon as it gets to 4000 rpm it stops accelerating. I can't get it past 5000 rpm. I can't really explain it, it's just like the car just stops pulling and literally wont go any faster past those rpm's. It sounds like the turbo is still spooling and I can hear the blow off valves when I shift. Also, the clutch is still really tight, no grinding or anything. Someone please help me I know nothing about these cars. A friend of mine said it might be because its not getting enough air? Please help.

[harward2420]

Oh, also, I know that there was some sort of (trying my best to know) sensor that was replaced in the engine. Not the sensor, but the part that holds the sensor in (which was some-what small)....it was located on the right side of the engine just about half way down if that helps. And there was something to do with a plug? Needed to be replaced...dont really know...