[Maxime Marquis] “The only way to arrange this is to erase all the keyframes of the problematic section, and the one following, and keyframing again.”
There is another issue with keyframes that I feel should be mentioned (and may or may not have anything to do with what you’re describing).
Linear keyframes used to behave differently (sorry, I won’t be able to give you exact version-specificity here). In the old days, when you made a linear (default) keyframe, you could rely on the motion to be perfectly, well, linear. Nowadays, you may find that sometimes there is some floating going on close to the keyframes, and that the motion has curves to it. This graphic shows you what linear keyframes look like when applied to a solid position:

This can be seen only when you have “Motion Paths” selected in the View Options. The solution below requires this also. When dealing with camera animation, you need to switch from “Active Camera” view to another view, like “Front” (or another camera).
My favorite way of dealing with this, when I want to be sure I have true linear movement (no curving, easing, etc.) is to do the following (Windows users, please excuse the mac-specific key command here; I’m sure you can work out the equivalent): Float your cursor over the keyframe in the Composition window, hold down Command-Option and notice the cursor changes to the bezier control/break tool. If you click on the keyframe, the bezier curves will ‘break’ and become perfectly linear.