1998 Mercury Sable Heat Issues?

The heat on this Sable works, but not too great. If you keep it on a low or medium setting, the heat will blow warm air, but if you put it on high the air is not so warm. Coolant isn't low, and if it is blowing some heat in the first place is doesn't seem that the heater core would be clogged (but what do I know). Any opinions? Thanks!