My character falls in unity 3D

I have created a capsule character and I move it using MovePosition method and rotate it using MoveRatation. That is, the movement is realized with the help of physics, but about 10 seconds after starting the game he starts to fall, although the first seconds I move calmly. Also I do not like that the character falls when colliding with other objects. I do not understand how to fix it, please help me.

Vector3 rotation = Vector3.up * hInput;
Quaternion angleRot = Quaternion.Euler(rotation * Time.fixedDeltaTime);
_rb.MovePosition(this.transform.position + this.transform.forward * vInput * 
 Time.fixedDeltaTime);
_rb.MoveRotation(_rb.rotation * angleRot);

This code is in FixedUpdate. hInput and vInput just check input "Horizontal" and "Vertical" and multiplied by some float values in Update

I don’t really know what’s happening, however I noticed you were using Time.fixedDeltaTime instead of Time.deltaTime. Maybe you could try changing this and that might fix it.

Your code looks fine to me, it should not have impact on gravity.
You can look directly at your Rigidbody and tick Is Kinemactic ☑ or untick UseGravity ☐ to stop your player from falling. However you won’t be able to jump anymore.