I just bought a 2007 Mazda 3 Hatch 2.3 Automatic, I love the car but has a issue that is shifting too early causing the vehicle to shake normally on 4th and 5th gear.

I said that is shifting too early for two reason:

1.) While cruising at 35mph the car is already shifting to 5th gear and the rpm are between 1600 to 1800.

2.) The vibration goes away when I press gas increasing the rpm over 2000.

It has to be something electric(sensor, selonoid) because the transmission shift very smoothly when Im driving a bit faster. Also when it is on manual mode which I have control over when to shift, the problem is not reproduce.

No Check Engine, AT lights are on.

Any suggestion guy, I'd appreciated.

Thank you,