Over the past year or two my wife's 02 xterra has been having trouble with power when climbing hills. It has 136k miles on it and I have done the complete tune up at 100k. It has had the tranny rebuilt as well but that has not fixed the problem. It also sound loud when accellerating, like the rpms are reving high but not effective. My gas mileage is unchanged, and it has no problems on level ground. I have replaced the fuel pump, all hoses, checked exhaust and replaced the converter. Ive put injector cleaner in it and had injectors cleaned and checked. Ive even switched to a higher grade gas. Im now at a loss. Any ideas?