You can take the tension off all the spokes, lay the wheel on a flat surface such as a garage floor, and see how much the wheel is bent. If it is substantially bent, get a new wheel. It almost may seem that somehow some spokes are loosening up. Try truing the wheel yourself and mark the spokes or the rim side that you are tightening and see if next time the wheel is out of true that it's in the same spot. It's possible the threaded nipples are stripped and won't hold the tension. It's also possible the spokes are being pulled through the rim, it doesn't take much. Maybe the spoke hole was drilled too large for the spoke. Maybe the spokes are defective and the metal is too soft and is being stretched too much and not "springing" back to maintain the tension. Again, if you mark the spokes or the side of the rim where you are tightening them, you'll see if it happens in the same spot.