Ok, know its clear why the calculation returns these weird values. But how i can compensate this time changes ?
Here's the calculation:
Code:
function speed()
{
   while(1) 
   {
     vec_set(Old_Rot, my.pan); 
     wait(1);
     vec_set(Now_Rot, my.pan);   
     vec_diff(Rot_Speed, Old_Rot, Now_Rot);
     wait(1);
   }
}

This first wait(1) is the responsible code line I guess. I need a always constant thing in this wait. Maybe something with time_step ?