I recently broke 100k miles on my 2000 GT Automatic and my engine has began to vibrate heavily (shaking the entire car) in the RPM range of 1200-1400. This does not occur while the car is sitting idle or when I am accelerating above 1500 RPM, or when I completely remove my foot from the gas and coast. In addition, if I am driving and have the gas slightly compressed at around 1600 or so RPM, I notice I will lose power and the RPM may dip into the vibrating range. The car was tuned up around recently, however no belts were replaced.

Any ideas?

It's a Mitsubishi they're going to fall apart eventually.

Check motor mounts and your transmission.

Bad transmission fluid?

Is it only a vibration or does it surge at all either? If there's surging then maybe a bad TPS sensor.

Does the vibration feel like it's coming from the tranny or engine?

Is your torque converter locked up at those rpms?

Does it do this in all gears or only in certain gears?