Movement Issue: x value of object resets automatically

I have recently been trying to build a 3rd person fighter action game after see many videos and tutorials. I have this issue with my script. My object seems to (while flying) reset itself after it has gone around 800 (on the x value of the object's position). I try to go to this distance again and then it resets (meaning the x value goes back to 0)....is tehre anyone who could help?