For the past few months, my 4Runner has been idling normal when I come to a stop but after about 20-60 seconds, it starts to fluctuate and will sometime shut off. I've done a throttle body cleaning, ran fuel additive, and ran a couple of tanks of premium gas but still the same thing.

Basically it idles at around 700 RPMs or around that normally. When it starts to act up, it drops to around 450 up to 500 down to 450, up to 550 down to 400 and then dies. These numbers aren't exact but give or take 50. If I tap the gas when it is fluctuating, it goes back to normal for a minute or so. Of course when i am driving, everything is fine.

I have about 174,000 miles on my 4Runner
Replaced plugs at 150,000
Replaced coil packs at 146,000
Timing belt was replaced at 74,200 - I wonder if this is the problemà
oil changed every 3000 - i know this has nothing to do with it.

Any help will be appreciated!