NHTSA Complaint #11514969 — 2019 GMC TERRAIN

I bought this car a couple of months ago. This problem didn't happen when I took it on a test drive from dealership. It happened after I bought it. Specifically when I start driving it more often. Sometimes a slipping occurs when shifting between gears 5-6. This happens intermittently on internal roads, not at high speed. When the car shifts the RPM goes up to 4000 RPM, sometimes even more, it feels like the car is in neutral for a few seconds, before it switches to the 6th gear. I have to remove my foot from the gas pedal to not let the RPM get very high. When this happens it feels like somebody hit us from the back. Last time my baby started crying when this happens. I'm a little concerned about our safety when we drive this car. The issue is that this problem is difficult to duplicate. Sometimes it happens more than once in 1 day and sometimes it doesn't happen for a while. I did some research online about similar cases with GM SUVs and some said that their problem was resolved after changing a defective valve Body and TCM and some said it's the clutch. It also can be transmission fluid but this car has a low mileage (28500) I doubt the transmission fluid need to be changed. Based on the GMC maintenance schedule, the transmission oil should be changed when this car reaches 45k. I took this car to the dealership multiple times, they check it and they tell me that they're unable to duplicate this issue. I'm not sure what else to do! I love this car. I'm disappointed that I had to deal with this issue that I believe is a safety hazard and could cause accidents. I need your help please!