Not sure what it is obviously :D Its difficult to say without seeing the animation. But things you can try:

1. Make sure that skip frames is not turned on.
2. Try to render out the animation (Image sequence) and combine it in a software that can do that to be sure that its actually lagging.
3. Check that its not a key frame issue, some wrong keys etc.

Changing the FPS from 24 to 30 shouldnt make a difference as far as I know, as its simply just how many frames a sec it uses. If its set to very low like 8 frames per second, the whole animation should look like stop motion, but since you say that its only at a given frame range it seems unlikely to me that it would be the problem. (But might be wrong) but you can easily test this if you render out the sequence and set the frame rate to 24 or 30 when combining them.