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Car stumbles below 3,000 RPM....no idea why.

1 May 2001
Point of No Return
So my little 1994 NSX must be mad at me for some reason, because it has cost me more time and money in the last 2 months than the last 2 years. It just turned 185,800 miles today, and for the past couple of days has had a nasty habit of stumbling when accelerating with the RPMS below the 2,500 - 3,000 RPM range. It is not throwing any codes now, but a few days ago threw a code '51' on me. I cleared it out, the code has never returned, and the car ran fine. Then while accelerating from a roll in 3rd gear at probably 2,000 RPM the car bogged down, so I shifted down to 2nd to accelerate away with a little stumbling still. The car has been doing this stumbling ever since. If I am above 3,000 RPM and floor it, it zips up to red line just like it should. I am baffled with where to start. The real pain is that I was going to register for NSXPO today (yes, I waited until the last day, I know), but now I am nervous to attempt to drive the thousands of miles with the car not 100% perfect.

Any help is appreciated. My buddy is just not running right. :(
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Got it!! Since there were no codes on this, I started thinking like the car was human. What would make me gasp or stumble? Lack of fluids or air. I checked my service records (this is WHY you keep service records) and found that the fuel filter was not very old, but could not find anything on a recent air filter. I pulled the air filter and the bottom was BLACK!! So I swapped in a spare (yes, I keep cabinets and attic space full of spare NSX parts in inventory at home) and went for a drive and tried to bog it down like it did before, and the car runs fine. Needless to say, I sure hope that I got it right, and that was all. My buddy (my car) is happy now, and so am I. :)
I think your stumbling problem will come back. Are you still on the original fuel pump?
Good point, I wanted to suggest that too as I ran into it myself but as the air filter was overdue I think the variety of possible faults is very, very wide. :)
fwiw Mitch my original fuel pump may be getting weak,my 96 has been giving me an intermitent lean code,and Larry B wants to measure fuel pressure.I've also thrown some techron fuel injector cleaner in the gas as per his suggestion.