Jerking while driving

I had my transmission filter changed and fluid filled about 2 weeks ago due to a hard shift between 1st and 2nd gear. It fixed it immediatly, and it ran and drove excellent. However, a few days ago, it has started jerking at about 45 mph. The rpm's will go up and down rapidly, and I think I hear a clicking sound. It doesnt do it all the time, as sometimes it still runs, and drives, and shifts perfectly. It will also, die occasionally at an idle, but starts back up immediatly, but I '2 foot' it to keep it running. What could possibly be the problem?