Transmission Problem

I need help, if anyone has any suggestions I would appreciate them.
I have a 2000 Elantra. Has about 68,000 miles on it.
Sometimes while driving the transmission shifts out of overdrive (4th gear) and only drive in 3rd gear, even when you come to a complete stop. The dang thing won't down shift at all, even if you manually shift it only goes in 1st and 3rd gears. When you turn the ignition off and start it back up it runs fine. I've had at the Hyundai service shop twice and they couldn't get it to happen. The last time I took it they found a code of the problem , but they cleared it out and couldn't remember the exact code. They drove around a few times ( at least that's what they told me ) and couldn't get it to mess up. Finally they told me that it was most likely the transmission going out.
Does anyone have any suggestions or experienced this problem?

When my 2000 Elantra Tranny started slipping etc. Had the fluid and filter changed and all worked well. I know it sounds a little to simple but it worked for me.

The problem that you're having with your car was something that I experienced in my '99 elantra wagon. Whenever I was driving on the highway at like 50 mph and I hit the accelerator to pass another vehicle I usually made the tranny downshift from 4th into 3rd sometimes even 2nd gear. I was accelerating fine until I came to a stop. (I got off the highway and hit the first lights) Once I began accelerating again the tranny didn't start at 1st gear. It began pulling at 3rd gear. My mechanic and the Hyundai mechanic said that the Transmission computer was going into safe-mode. The computer goes into safe-mode because either you don't have enough oil in the tranny, the tranny is overheating or the computer itself is malfunctioning. Basically if you're driving normally and the tranny all of a sudden goes into 3rd gear and doesn't shift back up or downshift then you are probably having a problem with the transmission computer. These transmission computers aren't expensive. Go to www.car-parts.com and you'll see for yourself that they are cheap. I changed mine and I never experienced the same problem again.

Another problem I used to have was when driving up a hill or mountain. While going up big hills when my tranny shifted into 3rd gear by itself or when I manually turned off overdrive with that little button on the shifter, the tranny kept driving on 3rd gear. It didn't want to shift back into 4th. This was basically another safe-mode problem my tranny was experiencing. All I did was instead of stopping I threw the tranny into neutral, turned off the engine, turned it back on and popped the tranny back into drive. The problem went away.