A few months ago my car started having this problem. When getting on the interstate, passing someone on the interstate or going up a steep hill, my car would jerk/slam into gear. It happened probably once a day. I took it to the Toyota dealership and they couldn't find anything wrong with it. My husband changed the spark plugs and the problem went away. Now, a few weeks later it started doing it again. What could this be? My transmission fluid looks great.