I'd suggest that you get VERY familiar with the Fanuc G76 threading canned cycle. A full description is in the Fanuc operator's manual.
With G76, you make all the threading passes with a one-block command. There are other "single pass" threading commands, but with these you have to manually program each pass. G76 will make a sequence of passes, and will finish the thread with a final "Finishing pass", all automatically. The G76 cycle also lets you specify the angle of the tool, so it can cut each pass so the chip is always on the same side of the tool.
Some small hints:
Start the threading cycle with the tool far enough away from the part so the Z axis servo has enough time to accelerate up to speed. Starting too close can cause small thread pitch errors on the first thread. Usually, .100 inch is enough.
Don't use CSS (Constant Surface Speed) while threading a taper. The spindle speed will change during the cutting pass, and the Z axis may not follow the speed increase exactly. CSS is turned off with a G97.
Make sure your spindle encoder is working properly. Most encoders are belt-driven off the spindle shaft, and these belts can get worn or loose. The spindle encoder has a "1-pulse per revolution" signal that triggers each threading pass. With a bad encoder, the G76 threading cycle may not work at all, or the pass may not line up with the previous passes.