2006 Grand Prix Supercharged Engine sputtering and etc.

I recently purchased a 2006 Grand
Prix GT 3.8L with a Supercharger.
The vehicle ran great on my test
drive, but after taking it home i
noticed that there is a slight chug /
sputter at low speeds (0-15) and at
some high speeds (40-45 and 50-55).

Now, I assumed that it may have
been bad gas, so I used up the
remaining gas that was in the car
when I got it, and filled it up with
brand new 93. This helped the
problem a bit, but it is still
occasionally chugging. There is no
check engine light on. Any ideas?

Additionally, the thermostat gauge
claims that the car is running cold-- I
assume this is a bad thermostat
sensor and intend on replacing it.
Could it be something else, you
think?

Thanks so much for your time