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?

[Box]

Checked the coolant temperature sensor for the ECM yet? Though that's usually going to rear its head right off the bat, same with MAP sensor. Only thing I can think of that'd take time to come on consistently after resetting would be EGR system related. With EGR the first time there's a fault it goes into temp memory, then if it happens again after the car has been shut off and then back on it triggers the light. Could be it's bugging out and opening the EGR too much/when it shouldn't which would cause a kind of bogging hesitation and potentially botch mileage. There's a giant troubleshooting procedure somewhere on the internet I had found when the EGR on mine was acting up so I'd only replace what's needed, but it was over two years ago but I'll try and find it.