Hi Federico,
I think the problem is originating from the way how the isotope script is trying to fit the items into the container. It changes the item height relatively to the change of the width, which I think is creating this overlay.
Luckily I think a min-width CSS property will do the trick, if set to the same as the original item height. Like so:
[code].isotopic .results .item {
min-height: 25px;
}[/code]