So a few months ago I got home, shut of the engine realized I forgot something, started it back up (maybe 10 seconds between off and start again) it REALLY lagged to crank and when it finally did it was idling below 500rpm and shaking badly. It's done it maybe about 3-4 times over the last few months and what I've noticed is it only happens when there is a short time between shut off and crank. Today's incident was about 5 minutes which is the longest time I've had it happen after. It drives normally accelerates normally but at stops it returns to about 500-600 rpm and shaking. This goes on for about 5-10 minutes of driving and then it returns to a normal idle never throws any CELs

Maintenance in the last year: plugs, wires, a/f sensor, O2 sensor, MAF, air filter, timing belt.

I do know my cats are poisoned but don't have emissions testing here and I'm averaging 17 mpg so until they start becoming an actual problem I'm keeping them

So the question is why would turning it off and then shortly back on cause a rough idle?

My thoughts would be either the intake air temperature sensor that's integrated in the MAF or your Idle Air Control valve on the underside of your throttle body.

If the air temperature sensor is reading hot air it will change the idle. But you did say you did maintenance on the MAF (cleaned or replaced?) so not too likely. One of my other Toyotas had this problem and when I swapped out the air flow meter it idled much smoother.

I'm not sure why the IAC would only act up at that time though. It still could be the issue but seems like it would either always idle bad when the engine is hot or not, not really a back and forth thing. Any chance your coolant is low? Maybe when you shut it off, the air settles at the IAC since it's the highest point? It's a stretch though...

Before you start replacing things for a rough idle. Try cleaning the throttle body. Run seafoam through the vacuum lines and fuel conditioner in the gas tank. Or you can go to a shop and get an air/fuel induction service.

550-650 RPMs is the normal idle speed. A clogged cat can do this to you as well.

I think the idle speed is a bit of a moving target. The FSM says 700 +/- 50, but I've know plenty that idle lower like what you are saying. Mine idles at about 600 but I think what the OP is trying to figure out is why it's changing. Mine always idles at 600 as long as it's warm, regardless if I just started it up or not. I think the OP is trying to figure out a drop in the idle.

Throttle body is right on, forgot that one. if there's excess buildup it may have also contaminated the IAC.

Checked the throttle body. NASTY!!! BAD NASTY!!! Looks like a project for the weekend

Thanks guys