I have a 1991 Honda accord ex. I can be driving a long and the sport mode light will start to flash, it also runs at 3500 rpms around 65 mph. After a period of time the check engine light will come on. If I stop and turn car off then right back on the light goes off. Also the speedometer does not work but when car dose run right it dose or will work some times. It feels as if it down shift but does not. What could be the problem? A sensor on the transmission itself or the computer?