I mash the throttle and it goes on it's happy way until around 5.5-5.7krpm. Then hesitates as if it had lack of fuel or air. I let off the gas until I'm around 1/4 into it, and the car accelerates to buzzer.
Any opinions?

I have adjusted the TPS and changed the inline fuel filter. I have not replaced the pre-filter or cleaned the tank.

Thanks in advance.
~suds

 

Check that your 6-ports are functioning
Check that the secondary throttle plates are opening correctly.
Check for vac leaks, or leaks in the intake train AFTER the MAF

A wideband would tell you exactly whats wrong with it.

But it definately is an Air Fuel problem and it just seems to me that its sucking air somewhere, i'll bet its going lean at 5k until you let off the throttle.

 

u got stock intake? wat about motor?

 

both are bone stock.
this is an s5, so the cat-back wouldn't affect the functionality(is that a word?) of the 6-port unless they aren't working.