Sync HD losing dropping lock

[kkfaudio]

Sync HD using Video REF.

Ref Present light staying lit, but Speed Cal and Locked lights fluctuate quite a bit, and will often disappear long enough and I get:

"The SYNC Clock Reference is not locked. Although the system will still play, it will not properly resolve until the clock is restored. Check your cables or change the Clock Reference or Clock Source."

It usually resolves itself quickly. The reference and loop sync cables seem fine - although I have not swapped the serial cable as yet, as it's hard to get to on the Mac. Is it possibly that cable, or is my Sync HD being faulty?

Thanks!