Code 25 - running lean, Cant figure out why its running lean. Options

[rapid36]

Hey Guys I have a 94 celica st and been fighting forever to try and get this thing to run right. I am getting a code 25 that I cannot figure out. according to the book its - Mixture control -continuously lean and possible causes are - Wiring, injector, H02S, intake/fuel ignition system, valve timing, ECM. I timed it according to the book which is 750rpm at idle and at operating temp with -10bcd. Then I replaced all spark plugs, wires, o2 sensor about a year ago, fuel filter, idle air control valve, I had another set of injectors which are the yellow top ones and so I put those on and still have the code, and the throttle position sensor today. The thing I dont get is when i reset the computer each time I replace a part the car runs fine for about 60 miles and my gas mileage is where it should be and i can go up small hills without down shifting, but after the check engine light comes back on i then have to down shift to get up the same smaller hills and gas mileage goes to about 25mpg. and only get about 260-280 on a full tank of gas. I also have a hesitation every time i step on the pedal ALL the way (flooring it) and this happens at no matter where I am in the rev range. Any ideas on what this can be?