Just completed this job last week.
I had exactly the same symptoms as you. New thermostat will sort you right out
The job is a pain in the balls. But very doable.
I have bruises all up and down my forearms from trying to do all the required work in the tiny space available
As said above,,,
Undertray off...
Boost pipe off...
Drain coolant from where the lower radiator hose meets the electric water pump under the car...
Auxiliary belt off...
Tensioner out...(optional)
Prize the alternator out with a pry bar....
There are three pipes attached to the thermostat...
The two rubber ones on top and bottom are simple enough to remove...
The metal one coming out of the front of the stat requires you to remove the two small triple square bolts holding it to the front of the engine block so that you can then wiggle it enough to get it off....
Remove the main stat...
Replace with new stat....
TOP TIP for reassembly;
Push the metal "bushes" on the alternator out a bit before you try and refit it. Otherwise it's way too tight a fit and you'll never get it back in. Don't worry the bolts will pull the bushes back into their correct position as you tighten them up.
Now enjoy your car behaving like a normal car because it actually has a working thermostat. Hallelujah
I found this video quite helpful.....
Hope all that helps.
Regards,
Dave