Have a 94 4Runner AT 4x4 that has two issues which started around the same time, so I'm not sure if they're related or not. Any help would be really appreciated!

First, the temp gauge fluctuates a lot while driving around town... most notably it will crawl up almost to the red line while I'm idling in gear but will then quickly drop back down to normal once I accelerate, especially to highway speeds. I don't think the the engine is actually that hot since it goes up and down so quickly, but why does it think it's getting hot?

Also, sometimes when I start the vehicle when it's already warm, it will idle super low to the point of stalling unless I keep the engine revved for a minute or two. After driving around it will usually level off at a normal idle, though sometimes when idling at a red light it will fluctuate between 3000rpm and 8000rpm as if it's struggling.

 

It is a good idea to clean all temp sensors- electrical or bi-metallic- every 20-something years .
Do not skip anything; clean them all, their connectors and their mounting, thoroughly to bare shiny metal..
Let us know what you find.