I've got a 1997 Mercury Cougar 4.6l V8 and it has started to idle low at about 600 RPM's when in gear, it dips to about 50 when i start to come to a stop and when i stay stopped without giving it any gas it dies. It seems to be accelerating normal and runnig normal on the highway it just in the city when i have to stop at stop lights/signs and when i have to slow down for other drivers. I put some premium gas in and it seemed to idle a little better so i'm thinking its something in the fuel system.