2000 astro van

Good day: My 2 weel drive chevy van has a tranny problem. I believe it to be a sensor as this only happens when it is warm and I have driven for an
extended period (25 kms or more). What happens is after a stop the tranny (auto) all of a sudden shifts really rough out of low slightly rough out of second then is fine. If I pull over, shut the van off, wait a few seconds, all is well and god is good once again. Anyone have any suggestions as what sensor is gone and where I would find it? Thanks all.
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