Are you using SPDIf as an input into the focusrite? If so, and you have it set as the master then it's almost certain to be the culprit as it does not have a clock line like AES/EBU does as an electrical signal based digital audio hardware format. The focusrite will respond as a slave to the samples coming in on SPDIF even if they are not exactly paced at the right sample rate which can and does happen. Run the SMPTE potential interaction to ground as well. SMPTE deals in frames and PG Products used to brag about SMPTE time code compatability vs other products way back in the day.