2000 Honda Accord hesitation

My car basically has no power at higher RPM. When I try to pass someone, and the tranny shifts down for "passing gear, " it puts RPMs at about 3700 and up. With full throttle it stays right at the speed its at, no excelleration, sometimes surging or bouncing a little. Then when it shifts back into the lower gear it will finally excellerate, but only slowly, becuase its not the gear made for passing. Computer finds no fault codes. I recenlty installed a header, and wonder if I've confused the sensors somehow. However, I suspect the fuel pump is putting out inadequate pressure to maintain the high RPMs, is there something else I should look at?