I just purchased my sonata. While driving my

Possible Vehicle Speed Sensor (VSS) problem, or a loose connection at the VSS.
Is your Check Engine light on? Are there any fault codes in your computer?

it very well could be a loose connection. There is a vehicle speed sensor on the transmission that sends a signal to the speedometer. the connector terminals on the speed sensor may be loose. Or the sensor itself is defective but it sounds more like a loose connection. The sensor is mounted on the top of the trans. maybe you should go to your nearest auto zone and get a printout of the sensor location they have alldata.