Screenshots to illustrate the bug
Currently hovercards appear relative to the mouse cursor.
That means depending on the initial location of the cursor the hovercard is shown closer or farer away from the link (see first two screenshots in attachement).
In the second case (hovercard shown farer away from the link) this leads to an issue: There is some "dead space" between the linked area of the link and the linked are of the hovercard.
If one moves the mouse (not too fast) from link to hovercard in this case the hovercard dissappears before the cursor actually reaches it.
The easiest possibility to solve this issue would probably be to show hovercards always adjacent to the linked area (e.g. the hovercards tip touching the link)
(Tests above with Firefox 29.0.1 on Windows 7)