1999 Mustang V6, Engine Coolant Issue

I have a 1999 Mustang with the V6 engine. It is having a slight problem with the engine collant system. For some reason, it doesn't fully warm up all the way. The temp gauge still registers in the "normal" range, but when opperating, it's slightly cooler than it's supposed to be.

First thing that comes to mind would be, perhaps the thermostat is stuck open? No, I already replaced it. Secondly, the heater core may be blocked? No, I already replaced that also (a real ***** might I add). Thirdly, perhaps there's a blockage in one of the hoses or somewhere? Nope, I already completely flushed the system.

What else could cause this?