Egr
Sure enough.
A flaky EGR can cause all kinds of cold/hot rough high low smooth idle problems.
Do a search on the 'search' tab above. There was a rash of EGR threads just a couple of months ago.
With the car off, test the movement of the valve by watching it (if you can see it) while simulating vacuum (suck on the vacuum hose or a new, clean hose). If it doesn't move, it's definitely bad. If it moves, it may still be bad if the valve end is leaking. If it moves but doesn't seat fully every time, it's bad
If the EGR is causing your idle woes, the valve is almost certainly leaking and usually stuck tight. As big a PITA as it is, replacing it is worthwhile so long as you can make sure that's the source of your problem.
