character controller isGrounded issue when landing

First of all, sorry for my bad english. So thats the issue, when I land on the ground, the controller.isGrounded switchs between true and false, just SOMETIMES. everything is fine, i can walk and it will be grounded everytime to be honest I just want to know if its an issue of unity 5 or its my fault, if its unity problem then im fine, I just want to know pd: I apply gravity in EVERY update and then i call Move();