Does this sound like a vacuum leak or intake leak?
 

I've gotten really bad at diagnosing and fixing problems with my car because it very rarely ever gives me any problems.

Symptoms...

Power below 4000 RPM's feels normal, still safe to drive.
Power above 4000 RPM's is almost non-existent, enough to accelerate... but not very quickly.
Kinda stutters a bit, feels sluggish, won't rev past 7500 RPM's.
Makes a "ssshhhhhh" sound, kinda like a turbo; but without the turbo.

Also, before when I shifted at redline the car would give me a very audible backfire. Now it's very, very weak.

It kinda sounds to me like a vacuum leak, but I have no idea what kind of vacuum leak would affect the car so much. It seemed to affect the car instantaeously. As in I drove the car and it was fine, parked it, drove it again and it was running poorly.

I've also not ruled out the idea of an intake manifold gasket somewhere being blown out... but this confuses me because I wouldn't think an intake manifold gasket would just go out like that.

Any ideas? I haven't dug into it yet but from the top of the engine everything looks fine.

Oh yeah... and the car isn't throwing any codes at me. It reacted in a similar way a while back when I knocked a blue plug (can't remember what it was for) off of the wiring harness when I was doing some work to the car. The plug is on the right hand side of the engine.

It did very similar things; sluggish acceleration, no power up top, wouldn't rev past 7500 RPM's.

These is a diaphragm inside the ACV that gets old & fails.
I'm not sure if you can hear it, but the other symptoms fit.

Have you checked the spark plugs? Maybe they have gone bad...

i have my ports wired, and it sounds like what my car does if one of them falls and closes, at redline it begins to pop, if i can get it there, at about 7k rpm it like hits a brick wall and stops. so check your 5th and 6th ports also your vdi

hmmm....

I was also starting to consider it to be a fuel issue. I'm going to run a can of fuel system cleaner through it and see if that has any affect on it at all. If it does perhaps it's time for new injectors... or maybe the fuel pump just isn't working as hard as it should. Fuel filter may be clogged...

But all of those other things sound like possible culprits as well. Perhaps I'll wire open my sixth port actuators and see if that has any effect on the situation.

The ignition is fine, but it is time to get new spark plugs. Although I'm afraid to spend the money because I don't want to foul them out if the problem is that it's running too rich. Nothing a wire brush couldn't fix though.

Thanks for the input guys. :)

I don't think it would be the fuel filters if the problem suddenly showed up, but I'd replace them anyway. Maybe it's a problem with your secondaries since the problem shows up just after they should start firing.