I have a 2008 g6, 240,000 mi... shifts into all other gears just fine but wont stay in overdrive. It's been this way for about a year. Figured the transmission was going so just figured I'd let it go then take care of it. But nothing has changed. Go in overdrive when you take your foot off the gas, but as soon as you push the gas it just shifts back down.. any suggestions would be great..