primetime to astro
Albred,
Crosstalk does not cause skew. Crosstalk causes jitter. Skew is caused by different lengths of wires from clock source to clock feed. Uneven load and parasitic capacitance presented by domains at the clock feed also causes skew.
Blueant,
Check your clock distribution tree locally in domains and globally between domains. These domains probably present different load capacitances to the clock tree, resulting skew. One or more wires carrying the clock signal might be longer than the other. I refer a domain as an individual digital block that shares the same local clock. It is the same term used in ASIC textbook written by Michael Sebestian Smith (Hawaii) and Digital System Engineering textbook written by Dally Poulton (Stanford)