tuning or trans going bad
NO! I don't care if your "Check Engine" light, or "SES" light is NOT ILLUMINATED. You STILL need to plug a scanner in and check for codes. This might shock, and surprise, you.... There can be codes stored WITHOUT any warning lights on the dash

The fact that you can shut it off, and restart it, and the problem goes away tells us that this *potentially* is on the controls side.
If you have a scanner that will show data, hook it up and drive around until the car starts presenting the issue and then watch your data stream to see if the computer is commanding the late shift, and the trans is just following orders, OR, if the computer is commanding the gear shift and then the trans is delaying in following the order. This little test will put you right directly on top of the problem.
Trans could be getting hot and acting up. Has engine been running hotter than usual?? Who tuned it?









