[Kitten]

Never use OnPlayerUpdate for these kind of codes what i would do is create a simple stock like

pawn Код:

stock CheckRank(playerid)
{
    if(YourXPvar >= 50) // or == depends on how your system works.
    {
         RankedUp(playerid);
         PInfo[playerid][Rank] = 1;
    }
    return 1;
}

I'm not really sure about this but i'm sure you are using the EXP when he/she gets a kill if not do the same thing when that function gets called for that, if you are using it when he/she kills someone call CheckRank(killerid) on that person that kills another player.