vehicle slips out of drive when it snows but still says "d"

Driving in snow. 3rd lane on the highway. Engine starts revving but no "traction control" light. Shutting the engine off for awhile opening the door (coincidence?) reset the transmission, but it did the same thing every mile or so for 8 miles until I got to the dealership. If you ever lose complete forward motion in traffic with vehicle behind and next to you, it is the scariest thing I have experienced on the road. Brought it right to the dealership in Nashua NH who said "There is nothing on the computer stating that the transmission failed, you need snow tires". I told him the tires don't spin the engine revs like it is in neutral. He said "nope". I left. Sent an email to Hyundai who told me to go to another dealership. So now, what do I do: wait for it to snow, try to get video of the issue, and try to convince another dealership that this is a death trap? Why didn't Hyundai hold the original dealership accountable for not stepping up? Im not lying or wrong about this. I know the difference between spinning tires and a revving engine. Any ideas?