CEL, code P0171

I have a 1998 toyota tacoma, 2wd manual transmission, 2.4L. the check engine light has been on a while now, and i have been trying many things to solve this problem. starting with the least expensive solves. so far the fuel filter has been replaced, both front and rear o2 sensors, ive run fuel injector cleaners through it, and i beleive we did something with the MAF sensor as well, but cannot remember. it has no driving issues, sometimes i hear a noise in second gear when its at about 3000 RPMs, just before i shift. no wierd smells. no wierd gas mileage. after replacing fuel filter and sensors the light went off, for about 3 miles, and came back on at accelerating to get on the highway.