There might be a few options:
Caption P Style: white-space: nowrap;
The above would prevent character returns on the caption. But that might blow your table out of whack.
Caption P Style: height: 50px; display: block;
The above (or something similar) would set a static height for all caption elements, making single line captions taller than they need to be, but keeping the height of each cell the same, so there would be no differing heights causing the missing space on the taller ones.
Give those a try and let me know if something works.