Auto Trans overheat?
 

I'm the second owner. Since my mazda3 (2006 auto 2.3L) is for commuting, I normally drive via highway for about 15miles.
Since always I drive it early morning & evening, I don't need to use A/C... until now...

Today I drove my mazda3 to downtown, with many traffic light, also with A/C (Hot Texan Weather!)

While in downtown, it started to shift very roughly from 2 to 3 or 3 to 4. Also, did not shift down from 4 in slow speed.

After a while, suddenly... (just before I was going up to hiway.)

1) AT light on
2) tranny locked to '2' when it stopped.
3) manual mode doesn't work. (I used manual mode a little, due to rough shift.)

In the manual, it says 'If the automatic transaxle fluid (ATF) temperature becomes too high, there is the possibility that the transaxle will switch to automatic shift mode, canceling manual shift mode and turning off the gear position indicator illumination. This is a normal function to protect the AT.'. Might it be a just this issue?

After a while, (turning off the key and wait some minutes.) it came back to normal.

Do you think that it is a simply (?) overheat issue?

Maybe mechanics or dealers say ATF flush or similar. Can it solve the problem?

definitely sounds like an overheat issue - make sure your fluid levels are ok. and did the engine temp look steady when this happened?