I have had an intermittent rough idle for a long time. Not a bad enough problem to be real worried about, but enough to be annoying. Every so often, primarily on cold starts, it would hunt for the idle, dropping revs down to almost stall and then just kind of bouncing around. Pretty easily fixed by just revving it up to about 2k rpm for a minute and it would "catch" and settle in to a nice smooth idle around 700rpm or so. This has been happening for quite a while, at least a year, probably 2 or more. I have replaced the IAC valve, the EGR, the DPFE, cleaned the MAF several times, and the spark plugs, and it never went away (I thought after I swapped the EGR it went away but it did eventually come back). Before that, I replaced the PCV and notorious PCV hose, and have changed spark plug wires, fuel filter, and probably plenty of other little routine things along the way.
I just took my car to a friend's shop to get the timing belt done (i am at 162k, been pushing that one...) Get the car back and it was idling like crap all the time now, jumping/bucking and threatening to stall out. Different than the intermittent, easy to "fix" bad idle described above. I figure it must be due to the timing belt change so I take it back to the shop. He determines that the EGR solenoid (also called EGR Control Solenoid) is stuck open, and replaced it. Now it idles nice and smooth. I'm really hoping that this also fixes my long-term intermittent idle problem too. I'm hoping that it has been sticking just a little bit, dying a looong sloooow death, and that my longterm issue is fixed. Seems weird because it shouldn't open until the car is warmed up - I'm thinking maybe from the previous drive, when the engine was hot and EGR was open, some debris got caught in the poppet seat (I assume it's a poppet style solenoid valve?) as the valve was closing when the engine was shut off, so it was allowing just a little vacuum through to open the EGR diaphragm valve a little bit and causing a bad idle... I don't know, maybe a longshot, but it makes sense to me?! I'll know soon enough if my longterm issue is solved.
Just figured I'd throw out my experience in case it helps anyone else...