Engine nearly stalled at idle (seemingly one off occurrence)

I was sitting at a light in neutral (foot off clutch) today when my engine (3rz) RPM suddenly plunged to the point it nearly conked out.

The conditions that this happened were:
-Hot 90 degree day
-End of an 8 hour drive (all of it highway)
-AC had been on for hours (can drop RPM, but it never made it get close to conking out)
-Was in neutral
-Normal engine temperature
-Half tank of gas (so dodgy fuel seems unlikely)
-Wasn't turning the wheels (PS pump wasn't straining engine)
-Air filter is fairly new, fuel filter is original with 186k on it, MAF was last cleaned months ago

When I heard the engine studder and shake, I put the clutch in which did nothing. For 5 or 6 seconds it felt like it was about to stall, and for another 10 seconds it's idle would rise and then fall below normal RPM (normal is 700 RPM). After that I turned off the AC to help the engine, and by the next light it was normal.

My question is why did my engine operate fine for hours before, then suddenly drop RPM at a light?

I've never had issues idling even with the AC on

 

Does the OBD scanner show any codes?

 

No, nothing. Would an IAC cause intermittent problems like that

 

Could be a bunch of things. Could be a fuel pump issue starting. Erratic issues are a pita.

 

Sorry that I cannot offer any help but without additional data, it is hard to tell. At this point you might as well wait and see whether the issue resurfaces again.