Transmission Stuck
 

My 2000 Montero Sport is not shifting out of 3rd gear, it also slams into reverse and drive. After driving it for 6-7miles it will start shifting fine, although when you shut it off, and immediately start it back up, it is stuck in 3rd again. The transmission fluid and filter have already been changed. After which, the problem has been more severe. It was not showing any codes, except one for a 02 sensor, and has just recently shown a code for the transmission temperature sensor. It seems to be an electrical issue maybe? Solenoid? PCM? Please help!