[member]

It seems ok, but i've simplified it a bit. Try it now.
Before the countdown starts, the values should be as so:

pawn Code:

Tmin = [whatever minute you want];
Tsec = 0;

pawn Code:

public SetCountDown(minutes, seconds, Float:x, Float:y)
{
   Timer = TextDrawCreate(x, y, "00:00");
   TextDrawTextSize(Timer,636.000000,824.000000);
   TextDrawAlignment(Timer, 2);
   TextDrawFont(Timer,3);
   TextDrawLetterSize(Timer,0.499999,1.800000);
   TextDrawColor(Timer,0xffffffff);
   TextDrawSetProportional(Timer,2);
   TextDrawSetShadow(Timer,1);
   TextDrawSetOutline(Timer, 1);

   Tmin = minutes;
   Tsec = seconds; // should be "0" if you want the whole minute

   TTimer = SetTimer("TimerSet",1000,1);
   return 1;
}

Here is the "TimerSet" callback that is repeated 1000 ms

public TimerSet()
{
    new string[5];
    Tsec--;
    if(Tsec <= 0)
    {
        Tsec = 59;
        Tmin--;
        if(Tmin <= 0)
        {
            OnCountDownEnd(); // Time's Up
        }
    }
    format(string, 5, "%d:%d",Tmin, Tsec);
    TextDrawSetString(Timer, string);
    ///TextDrawShowForAll ?
}

Also, aren't you supposed to be showing the textdraw once you've formatted the string? maybe the textdraw is being shown every 10 seconds in another timer or callback.