After shut off boiling.

What causes it, and how do I fix it?
Anyone else have this problem?

If you are referring to removing the coolant cap and it's boiling, that is normal. (Unless you have an inconsistent mix of coolant to water ratio).
If you are referring to you shutting off the engine and leaving the key on reds then the tempurature gauge shoots up, then you have another issue. Sounding electrical issue (temperature gauge earthing on reds).
Good care for your cooling system; If you replace the thermostat, have a radiator with no leaks (even small leaks can cause issues), flush / clean the cooling system (age of the system needs a little love), ensure no air in the system (follow the BGB to bleeding the air), and use Toyota red coolant at the recommended ratio; you shouldn't have heat issues....
I have driven hard on 40 plus degree day for hours, to the point of the aftermarket manifold cracking, however temperature gauge / temp is fine.

I do have all the heat items in place like intercooler fan on, rain shield removed (don't drive in the rain), heat shield on manifold, wrapping on intercooler pipes, larger intercooler.