98 Explorer rough idle no codes

My 98 Explorer has started to idle rough, the rpms drop and then rise, it has stalled once, it doesn't do this consistantly.
23k on the vehicle and the engine seems smooth while driving, the problem only seems to occur during idle say at a light or stop and also when the transmission is in park, doesn't seem to matter in park or in drive.
There are no computer codes.
Any ideas please?

my 97 4.0 sohc was doing the same thing.i replaced the egr valve yesterday.mada a bige difference.got it from bumper to bumper for $45.00.try it.