Time for a fluid and filter change...
Check the fluid level. It sounds like the trans works better when the fluid is warm and has expanded. Modern transmission like yours do not like low fluid levels and will thank you by dying and costing you a lot of money. Like joemom say - change the fluid and filter and use the correct and expensive synthetic stuff, not the generic stuff the oil change place will try and use.
Check the level of the fluid when the engine is cold. If the dipstick has a "cold" level mark, make sure it is within the range. If low, add just a little at a time to bring the level up. If the stick has no cold level then the level should be toward the bottom of the hash marks on the stick. Now drive the vehicle a good while until the engine warms up well. The transmission fluid should be hot, not warm. When hot, check the level again making sure it falls within the hash mark level. Overfilling the transmission can cause shifting problems. If it is overfilled, removed via siphon to bring the level down.
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