Hi, I recently purchased a 2000 Silverado 1500 5.3l with an automatic transmission from the original owner (older guy in his late 60s)

the truck has 280k miles and the original owner claims he never had to do anything to it other than standard maintenance. She runs like new and transmission shifts perfectly smooth.

But what I noticed a few days ago after driving on the highway for around 30 minutes in 90 degree weather, as I got off the exit the shifts from 1-2 and 2-3 where hard shifts. So today I got back on the freeway and the same thing happened. So I turned the truck off for around 10 seconds and started her back up and she was shifting nice and smooth again.

Any advice would be high appreciated!

 

check the transfluid condition and level.
check for dtc set in the transmission control module and ecm