Over the last week my Accent will idle up from 900 rpm to 1500 rpm on its own after start and continues this while in park. It will not just stay at 1500 rpm, it moves up and down from there, it will return to 900 for a few seconds then act up again. This problem does not happen all the time. If you step on the throttle and let go it takes it about 5 seconds to return to 900 rpm idle. When you are running down the highway at better than 50 mph, the transmission will move out of lockup to passing position. It does not matter if you are going up hill or down hill, or if you step on the throttle or let off of it. This is also not all the time. I think the idle problem is tell the computer that I have stepped on the gas so it is shifting the transmission back and forth. Their also is no codes coming out of the computer. Anyone had this or know about it?

 

Try checking the TPS (throttle position sensor) on the side of the throttle body. That's where I would start. Then check the IAC (idle air control) valve, also on the throttle body.