Haven't tinkered with the Power FC no. It really only happened last night.
 
It's probably still more likely thremo stat related because if I sit at a traffic light, it will get to 84deg, but then if i start moving quickly it'll start to drop.

Make sure you put a genuine Toyota one in. The aftermarket ones are ****e and don't work well.

might have to buy some TRD ones frmo japan to keep in stock after this lol.

But trd ones will run colder around 65deg no? Not what you want for street imho.

You don't want a lower thermostat. Would drive like the car hasn't warmed up. I'm a tad suss about what's happening with yours & hope it is just the thermostat.

When's the last time it was changed?

Are Tridon thermostats still garbage??  Back in the day, every one I fitted would cock over in the plunger making the plate misalign and get stuck cracked open leading to the exact symptom OP describes.
 
I found that Waxstat ones were as good as OEM though.  They have a dished poppet plate that guides smoothly into the seat and never jam up.
 
Another plus with OEM is that the supplied gasket/rubber seal tends to be a LOT better quality.

Never changed it before Jay, so over 10 years!

Surely you've had a cooling system service in that time with new thermostat, rad cap etc? How many k's in 10yrs?