Why does my 1997 3000gt SOHC abruptly downshift on its own?

Your 1997 3000GT SOHC may abruptly downshift due to several potential issues, including a malfunctioning throttle position sensor, a faulty transmission control module, or low Transmission Fluid levels. These problems can cause the vehicle's computer to misinterpret the vehicle's speed or engine load, leading to unexpected gear changes. Additionally, worn transmission components or a slipping transmission could also contribute to erratic shifting behavior. It's advisable to have the vehicle diagnosed by a professional to pinpoint the exact cause.