Overheating car

Does anyone know why a car would overheat
intermittently, have a rather unsteady
temperature gauge (it bounces around and
NEVER stays in one spot or sits at the middle
no matter how much I drive it) and would be
blowing out cold air with the heater turned
on? I replaced the lower intake because it had
a crack in it. Replaced the temperature
sensor, the thermostat, had the coolant
flushed and oil changed and the engine still
was running rather hot. A few weeks went by
and the check engine light came on saying
the coolant fans needed replaced... so I paid
$560 for that repair. Ran fine another couple
weeks but the gauge was still fluctuating
crazy and still running hot. I could be driving
70 mph on the interstate and some times it
would say the engine temp was only 150 and
I’d been driving 15 minutes. Other times by 15
mins it would be past the middle mark and
going up To like 200-220 and falling back
down to middle. I started needing the heat
turned on because Its getting chilly outside
and I noticed the heat stops running hot and
starts blowing out cold air and then the car
temp gauge will rise. I’m at a loss. I took it
back to the shop and they told me it was air in
the coolant system and they took care of it.
Long story short, it didn’t and when I took it
back to the shop, they told me that the 3.4
liter engine in my car is basically crap. It’s still
doing the same thing and I’m running out of
help with it. I’ve spent $2000 that I just don’t
have and it’s not fixed. Anyone have any
suggestions on what it could be? I might also
add that there is no leaks. No squealing. No
smoke. Every time it’s been looked at I’ve
been told it’s in great condition. but obviously
something is STILL wrong. Its only been in the
red twice and both times I immediately pulled
over and shut the car off completely and had
it towed. Anyone have any suggestions on
what it COULD be? I’m taking it to a Chevy
dealership in a couple days to be looked at.
But thought I might come here for help as
well.