I haven't had the problem, but from reading other posts, it sounds similar to the symptoms of a dying "PNP" switch. Can you temporarily "cure" it by moving the shift lever through its full travel several times? (From "P" to the other end.)

The switch is on the passenger's side of the transmission with the exhaust system partially hiding it. It tells the transmission ECU where the shift lever is positioned. The switch is connected to the electricals at one of the plugs on the driver's side. The switch is expensive and the exhaust system makes changing it a b#$@h, I'm led to believe. People have said they changed it without moving the exhaust but I don't think I'd like to try it on my car.