I've had the subject problem for a few months now. I've replaced the fuel filter, plugs & wires, timing belt, mass air flow sensor, and the oxygen sensor. I have a TRD S/C that's been on for about 78K of my 93K miles. I contacted Gadget @ Underdog Racing to see if fuel delivery could be the problem starting this late in the S/C life. Any ideas would be greatly appreciated. I was now wondering about the fuel pump and pressure regulator.

I was going to guess the throttle position sensor, but that messes up a range of the pedal's position when it dies, not a range of rpm's.

Not sure but maybe your O2 sensor connections are not ideal. I had this problem on a honda CRX, replaced the O2 sensor, it was still surging. Then after racking my brain, I re did the crimping on the o2 sensor b/c I wasnt sure they made good contact. And wala, it took care of it. Worth a try I guess. (get good crimpers, not thoese cheapies you get at autoparts stores. lots of people in the car audio sceene make the mistake of cheap crimpers.)

So now I've replace the 02 sensor, then the MAF sensor (first cleaning the original, then replacing), plugs & wires, fuel filter, and timing belt. I took it into the dealer for a diagnostic session and they spent 3.5 hrs and only charged 2 because the forman was stumped! I did have the ck eng light come on now and its giving a missfire and cylynder #2 missing code. I also notice a fair amount of oil and crud in the throttle body when I cleaned it. One thing the dealer did was lower the idle and now there's a noticeable miss. About at my witts end here. Still low power, surging @ 2400-2600 rpm, then @ 3400 rmp a racket starts that almost sounds like valves, but, the dealer informed me the the SC tensioner was bad - I replaced it and it still makes that sound and appears to loose ALL power at that point.

Thanks twodoorunner. I'll check the wiring on both sensors.