Camera Move While Dollying

Hi-

Hope someone can help.

The question is: why does the speed of a camera moving along the X axis vary when you drop 2 keyframes in the middle of the move to adjust another parameter? And how can you prevent that?

I'm trying to have a camera tilt while dollying at constant speed next to a man walking. Here's what it should do, all while dollying:

1. Camera is steady on man's feet.

2. Camera tilts up to man's head.

3. Camera is steady on man's head.

To do this, I set start and stop keyframes (#1 and #4) for the man walking and the camera dollying -- both along the X axis only, and the distances equal. At the start, the camera is on the man's feet. At the end, it's on his head. This works to have the camera gradually tilt the whole time.

Next, I add a camera keyframe (#2) between #1 and #4. I change its tilt angle to be equal to the tilt angle of #1. This works to have the camera start on his feet, then begin to tilt at #2.

Finally, I add a camera keyframe (#3) between #2 and #4. I change its tilt angle to be equal to the tilt angle of #4. This works to have the camera tilt as I'd like. But a problem occurs: the speed of the camera moving along the X axis now varies, and the man goes in and out of the frame.

I tried fiddling with the slider that pops up when you click on a keyframe. It helps slightly, but there's no position where the camera's speed along the X axis remains constant.

Any ideas?

Thanks!!