Toyota Repair: 2000 Camry Rough Idle, valve cover gasket, crankshaft sensor

Question
I have had an ongoing issue with my 2000 Camry 4cl that has stumped me, time to ask the experts. I am by no means a mechanic, but I like to think I know my way around a Toyota pretty well. The car has a very low idle when started, almost stalls. When driving at normal speeds it runs fine, but when stopping at a red light or going in reverse it idles low again and stalls. I have gotten no error codes besides some random cylinder misfires occasionally, and I have replaced/repaired/tested the following:
-Timing Belt Replaced
-Crankshaft Sensor Replaced
-Camshaft Sensor Replaced
-Valve cover gasket Replaced
-Plugs/Wires replaced (Correctly Gapped)
-Air and Fuel Filters replaced
-EGR Valve/Diaphragm tested
-PCV Valve replaced
-VSV tested (12v relay fires correctly)
-Idle Air Control Motor Cleaned and tested
-Reset ECM idle (by disconnecting battery)
-Cleaned/tested butterfly valve intake throttle body
-Checked for vacuum leaks/clogs
-Cleaned injectors/Replaced gas
Replacing the plug wires and resetting the ECM idle afterward helped for a few days, now it is back to the same issue. The coil packs look OK but are very expensive to replace. I have also considered a dirty manifold as a possible issue, but unlike the older model cars I know it cannot simply be removed for testing purposes. It is not smoking or showing any obvious signs of sludge buildup. With no error codes being thrown, can you recommend any other possible causes?

Answer
This stalling and low idle problem is usually caused by a sticking idle air control valvebut it seems that it checked out ok, you mentioned a single cylinder misfire code at times, do you have the code number? It could be a bad coil pack which is kind of common and they can be itnermittent, a code P0301 through 304 would be a misfire code, the last digit indicates which cylinder is misfiring, another cause but not as common is a intermittently sticking injector.