1997 Taurus rough idle

I have a 1997 Taurus.3.0 OHV 6 cylinder auto w/102,000 mi. It is idling rough and stumbling at red lights, sometimes dying. It restarts fine. It also surges, then idles back down when in park. All this happens after car is at operating temp. I am a second owner. The car has had egr, egr sensor, cam sensor replaced. Fuel pump replaced before I bought it for my son. Now I'm leary of letting him drive it. The only code it shows is a P0175. Rich on bank 2.I pulled plugs on bank 2. Cyl.1, 2, &3 show no signs of carbon deposits which I thought would be evident of rich mixture. Also can find no evidence of a vac leak. Any suggestion on what to repair or replace would be appreciated.