i have a 2001 elantra with 136k. the automatic transmission (which otherwise shifts very smoothly and shows no sign of problems) shifts suddenly and erratically when i'm driving at a constant speed. i'll be traveling along (speed doesn't matter), and it will suddenly downshift, then upshift, then downshift, etc. this started off being very occasional, but now it randomly shifts every 5-10 seconds.
i've changed and checked the fluid, and tried replacing the Input Speed Sensor and the Output Speed sensor...that didn't fix it. it also does not blow a check engine code. i'm convinced there must be a sensor somewhere that is faulty. it really seems as though the transmission is electronically confused.
any help is greatly appreciated.