2005 Nissan Sentra • 160,000 miles

I am trying to find out why my vehicle is sluggish on acceleration and slow to shift gears at times. I have changed my oil and oil filter and checked all the fluids and can't see an immediate problem. Also, recently my check engine light came on which I got Autozone to check the code which was P0128 - below temp and I then noticed that when I had the heat on my gage showed completely cold, when I turned the heat off, the gage went back to the middle. I assume I need a new thermostat but I am more concerned of the way that it is driving. Help!
January 9, 2013.

If the engine doesn't warm up properly, it will run poorly. I would recommend replacing the thermostat and then see if it runs better. Chances are it will.