Have a passenger hold, or tape a clear glass of water to your dash, fill it half full, and take a video of the glass when it's happening. Try to show the road surface (showing a smooth road), along with the odometer reading and the VIN, and even get out of your car with the video still rolling to show a better view of the road. That's about the only way you can at least prove the problem exists, and that you're under warranty, and that it's your car that you're videoing.
Eventually they will sort these things out, but it may be important now and/or in the future to have that video.
I've had an insane number of problems with mine, but I've taken to documenting everything really well, and my situation is going to get handled, one way or another. But I know, as someone who is married and who also works on cars, it's really hard to figure out what the problem is when you can't see it, feel it, or in any way experience it first hand. I can't blame mechanics for that sort of thing, but I can prove it's 'real'. I can't tell you how many times I've had my wife say 'the car is doing this', and then it doesn't do it for me, and I just keep hearing about it... With new cars, until someone with the same issue gets it solved, it's tough. That's first model years for you. I hope you get it resolved soon, though.