[Dylan Hargreaves]"Maybe something to do with keyframe interpolation?"
Kinda-sorta. John's recommendation to turn keyframes to Linear will definitely work, but it may not be the look you want.
If you still have the Bezier keyframes you started with,check out the Motion Path Handles on each keyframe. You may simply have to get the motion path squared away to get the movement you wanted. Check out Aharon Rabinowitz' two podcasts about the Boomerang Effect for more info.
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA