BMW: Fuel Issue on a 91 325IX, fuel pressure regulator, filter performance

Question
I'm having a loss of power after 5,000 rpm's in 3rd gear and above. I've tried to fix it by changing the following, spark plugs, fuel filter, fuel pressure regulator, catalytic converter, air filter, performance chip. they helped but didn't fix the problem completely. Do you have any other ideas?

Answer
Matt,
 When you say "loss of power" do you mean the car stops pulling as hard as it was?  Or do you mean that the car stutters like you hit the rev-limiter?

 I've heard of this 4500rpm invisible redline before, but I've never encountered it myself.  To my knowledge no one has found the fix either.  It plagues several cars.

 If it was the first problem, then you might not have enough fuel pressure to feed the engine at high RPM's.  The E30's fuel system is very week.  From the factory it will only support 170hp.  That means that the injector capacity will only flow enough to properly run a 170hp 6-cylinder.  Now these engines make about 168hp in stock form, so you see the problem!  If you do anything (chip for example) to modify the engine/car to produce more HP, you will be out of fuel.  Then when you compound the fact that the vehicle is 15 years old, the fuel system probably isn't pumping the same pressure it was when it was new.  Then you have injectors that are likely dirty and you are at a real fuel shortage at high rpm.  SO to remedy this problem you can install a new (preferably upgraded) fuel pump to help deliver fuel.  Walbro makes some good pumps, and most E30 guys use them.  Read up on this website www.e30tech.com

 Good luck
 Josh