2000 E150 question

I just purchased a 200 E150 conversion van with 101,000 miles on it. The problem I'm having is that the van seems to jerk back and forth like the transmission is slipping or something. I'm not certain how to describe it. It mostly happens when going up or down hill and even of minor inclines. Any help or ideas on what the issue could be would be most appreciated!

The trans is "chattering" in and out of overdrive. I used a Lucas trans additive that helped. I intend to change the fluid and filter soon.