Why does my nav. screen come on (at startup) most times, but not all times? And when it doesn't come on NOTHING works to get it to come on except to shut off the ignition and try again....? Very annoying.

Also, is 14MPG considered avarage mileage for this vehicle? I just bought it, replaced the oil, filter, air filter and tire pressures are good, and no stored codes....it is a 2004 all-wheel drive and has 65K miles on it. Should I change the spark plugs? Anything I can do to improve the mileage? Thanks