Check you fluid level. My car has occasionally had the problem when I come to a stop. It feels like it's in neutral. I just wait a second or two, and the fluid comes back up to pressure & everything is fine.
Most typical when the tranny is cold first thing in the morning, or if I stop hard.
One of the best solutions is the addition of an external tranny cooler. Not only does the addition of a stacked plate cooler potentially double the service life of the tranny, but the added fluid volume practically eliminates the problem.
