2005 Nissan Sentra Trying Find Out Vehicle Sluggish On Acceleration Slow Shift

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!