My Elantra 2001 (265K) automatic Transmission (original) randomly slips to to lower gear specially when going on high speeds e.g. 50 mph. I have to stop, then it goes to normal there after, it used to happen every few hundred miles, but now it's happening more frequent. Otherwise, shifting is smooth on all gears. Is that a transmission overhaul job, or maybe a sensor (e.g. speed sensor) or accessory part replacement. I've checked the oil level which is ok. I also drain and refill the transmission oil every 15-20 K
any ideas my self for the past 100 K (used to do a transmission flush at the shop).
Thanks in advance