The intake manifold gasket on my 2000 Rodeo went at about 40K. Sounds about right for the symptoms. Suprised the dealer didn't check for it as it is the most common cause of erratic idle. Also might want to check your plugs again - I went to NGK iridiums and my idle became pretty erratic and rough, so I went back to the original Champs and it runs as good as the day I drove it off the lot. Also check the gaps on the plugs again.

Rob