Creating that slow/smooth photo sliding effect; how?

Hi, I've created a Flash movie, using 8, publishing to 7, at 12fps using about 1200 frames, sliding a line of photos in my 500-px wide document, at about 400 frames for each photo to slide over to the next one.

They slide over, but jerkily. I've seen this effect a bunch of times, and photos slide over slowly and smoothly and beautifully.

Is there a secret to having that effect happen? I tried using up to 36fps, to no really noticeably more smooth sliding of the photos. Do I need to make it some way- really-high # frs, or is there a different secret?

My photos are all just separate symbols, each dragged down into keyframe 1, then slid over for each next keyframe, 400 frames apart from each other.

Thanks for any suggestions!