I have a 1999 Subaru Outback Wagon with 107K miles and have had this problem intermittently now for a few years. While in the process of slowing down to stop the car sometimes starts to shake and then stalls. It will start up again right away. I have taken it to the dealer numerous times to repeat the problem for them, but it fails to do it again. With no diagnostic code they are at a loss as to why it keeps doing it. It will stall when coming to a stop after traveling a short distance or after traveling a long distance down the highway and slowing down in the toll lane.
It sounds to me like your transmission is not downshifting. An alternate solution to verify the problem would be to shift into neutral during probable occurrence. I would have the transmission checked, and fluid and filter if it is time.