Another CEL on 2001 Accent

Sometimes when I drive my 2001 Accent in stop-n-go traffic, the car will make a thunk sound (transmission noise) on deceleration and just before a full stop. After, this happens the acceleration is jerky and
the CEL comes on. Also when the traffic frees up, I notice the car is stuck in a low gear and the engine revs high (4000 rpm).
I turn the car off. The next time I use the car, things are back to normal and the CEL goes off.
What is going on?
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Your transmission computer has seen a problem. You'll need to check the DTCs in the transmission and engine computers to gather information as to what was seen.
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