From a stop hard shfting from 1st to 2nd then softer shifting to other gears. When the vehicle comes to a stop and restarted, shifting becomes normall in all gears until it happens again sometime later. It could be days, weeks later in any kind of weather or road conditions.The last time was about 11 months ago, at that time a GM dealer could not resolve the problem.
Given all the information you posted about the car all I can tell you is you likely have a transmission problem. But you knew that.
The dealer had the car so they knew what engine and transmission were in it, how many miles were on it, if a check engine light was on, the error codes stored and how the fluid looked and smelled and could not identify the problem.
You have posted none of this information and expect us to diagnose the problem. Sorry, we need more information.
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Off the top off my head, I’d guess the pressure control solenoid is at fault. Same issue happens with my 2005 lesabre. I imagine it’s the same 4t65e transmission. Should set a fault code. No check engine light, but a code can be retrieved with a scanner.
185k miles on mine. I changed the fluid and filter a couple of times (which didn’t help), then decided to ignore the occasional hard shifts.
These are the DTC’s for the transmission when the Check Engine light turns on.
https://www.transmissionrepaircostguide.com/transmission-diagnostic-trouble-codes-dtc/
Tester