2008 Cadillac Cts 3.6L V6 Overheating issues
 

For a few months now, my car's temperature has been acting up...During the day especially, rarely at night. I live in Florida so it's super hot out in the Summer. When it's driving good, it's good, but suddenly the temp will start to rise, usually while I'm idling at stop light or started my car a few minutes before actually leaving etc..the temp will go up, but sometimes if I keep driving and increase speed a little bit, the temp goes back to normal , other times it just keeps rising until I have to pull over.
Sometimes the resovoir will be smoking white smoke and the car will be making that whistle sound it makes when overheating and other times it does it won't
When I unscrew the cap on the resovoir, the water/coolant is bubbling pretty bad, and it's rust colored, which is odd since the resovoir is made of black plastic..

I fill it up with water or coolant(which lever I have at the moment) and wait about 15-20 mins to start it up again, usually it's fine after that, but other times, like all afternoon today, I'll start it up and drive not even 5 mins before the temo starts rising again, faster and I have to pull over again m This happened today around 6 times in a row, literally ...I was pulled over all afternoon almost ...This was the 1st time it overheated so much and so many times in a row..

it's been happening more often lately ...
I don't have the money to take it to a repair shop to figure out what's wrong and then to pay them to fix it so I'm trying to buy parts adn fix myself with my friend's help .

I was thinking probably needs a new thermostat, coolant reservoir and possibly water pump ...
then I read it could also be head gasket ..

I don't know where to start ..any advice is greatly appreciated !
thanks
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