Hyundai Repair: 2000 Sonata 6 cyl auto transmission, transmission control module tcm, transmission control module

Question
On hot days (most days lately) my transmission gets locked into 3rd gear and stys there until I turn off the motor. I've tried changing the transmission fluid but to no avail.

I've had it at several automtic 'specialist' but they can't find the problem. To make things worse the car is beggining to stall when taking off but fixes itself when I re-start the motor.

Any idea on a solution or reputable repairer in the Western Melb suburbs?

Answer
I don't have much to offer about reputable repair shops in Australia as I've never been there, but it sounds as if you need to find someone capable of reading both the engine and transmission trouble codes.  Typically, when the transmission shifts to third gear and stays there, it's because the transmission control module (TCM) has seen some sort of issue.  Normally, the check engine lamp will come on in conjunction with the transmission locking in third gear.  Common problems causing this to happen are failures of either the input or output speed sensor.  Ultimately, it'll be the transmission trouble code that will give you information about where the fault may lie.  

If you can obtain the trouble code yourself, report it here and I'll guide you on the next steps.