2004 Corolla Dropping Into First Gear

#1
2004 corolla, 85k miles. Automatic trans will unexpectedly drop into first gear. After a few miles it will self-correct and resume normal operation. This happens only once every few months. Dealer checked computer codes, etc. and found nothing wrong. Problem would not repeat when dealer technician took if for test drive. When problem first appeared, I did a drain/fill twice to
replenish the trans fluid. Dealer checked fluid ok. Any ideas??
Thanks.
 
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