overheating problem?

I just got an 05 Chrysler 300 from my parents and
when I got it 2 out of 6 cylinders were misfiring.
(Boyfriend replaced all 6 plugs). Just recently my
temperature gauge has been going nuts. Running
cold, running hot, fluctuating when at idle. It never
really reaches the red and when it does it doesn't
stay there long. Usually when driving it will stay
normal, right in the middle and if it starts to get hot
I turn on the ac and it fixes itself. If I turn the heater
on it used to fix it too, but it often blows out cold
now, especially when stopped. I just changed a
bunch of parts on it so that might give you a better
idea. Any help will be appreciated.
-thermostate changed x2 in the last 7 months
-radiator changed 8 months ago
-6 new spark plugs

Also, I've read somewhere that it might be a head
gasket but my boyfriend said no because there is
no white smoke. *the computer at oriellys is
reading misfire in cyclinder 4 again though and
that guy said that might be because I didn't replace
the things that go with the spark plugs (sorry I
forget the name, coils, maybe).