Hyundai Repair: 2001 Hyundai Elantra, hyundai elantra, transmission control module

Question
Good Morning HT,

So I've seen one question similar to what I'm about to ask, but I don't know too much about cars and too busy with work to get my car to a shop until the weekend so I need some advice.

What happens with my car, 2001 Hyundai Elantra, is that when it attempts to switch gears in automatic drive, say from 1st to 2nd or 2nd to 3rd, half the time the car will jump and get locked in a certain gear.  I've also had the same thing happen as I slow down for say a red light and as the car lowers gears, 3rd to 2nd then 2nd to 1st, sometimes it'll even happen then.  I saw another post that said this happens between 30 and 50 mph and is a problem with speed sensors which seems about right, but it seems like a transmission problem to me.

But considering I'm not the expert, what is the problem here?  I only have a small window of time to bring my car to a shop on Saturday and would like to have knowledge of what's going on so some place doesn't try to rob me.

Thank you for your time and attention.

Answer
It sounds like the transmission control module is recognizing a problem and putting the transmission in failsafe, which is third gear only.  You'll need to have the diagnostic trouble code to know the nature of the problem.  Faults with the input and output speed sensors are the most common issues.