Use this script for clamping the angle. Works with any input values.

function ClampAngle (a : float, min : float, max : float) : float 
{
while (max < min) max += 360.0;
while (a > max) a -= 360.0;
while (a < min) a += 360.0;

if (a > max)
    {
    if (a - (max + min) * 0.5 < 180.0)
       return max;
    else
       return min;
    }
else
    return a;
}

It's not mine, I think I got it at UnifyCommunity. I also had problems when setting limits to MouseLook, but using this function for clamping the angle worked perfect.