What could be generating this waveform in an EMC Compliance test?
It looks like roughly 5MHz impulses with a resonance at 60MHz to me judging by the sidebands (20 per 10 MHz)  but could be a 60MHz clock with roughly 5MHz recursive sidebands.
The fastest way to find the offending signal is to use a small loop antenna probe to DSO or scope (Coil or wire in a small loop) to sniff out the near-field noise.  If you have 2 sized loops you can get a quick look then a smaller loop to get near the chip, trace etc. 60 MHz is about the size of a small room resonant frequency.
If indeed it is the data on the external cable then a Balun on the cable is a valid fix for that until you put a CM choke or SMT PHY on the data stream on the MOBO.
This is also the best practice for pre-test scanning the signature emissions before paying for the unintended radiation tests at 3m at an outside lab.  I would use a current loop probe for near-field inside the box and an E-field probe or just a wire on a coax end into the Spectrum analyzer for a broadband spectrum. Antenna efficiency drops for longer than 1/4 wavelengths but it is still broad spectrum.