Db9 shield connection? in plastic hood!
Wherever your problem is coming from , you have two obvious solutions, reduce the offending egress source with snubbers or improve the immunity of the so-called shielded balanced differential signal.

How this is done depends entirely on your analytic test skills to define the above threat levels.  

I would do this with 50MHz current probes and 200MHz FET diff probes. What you have may differ, but can still be done with ingenuity, such as connect unterminate shields with 50 Ohms and measure voltage drop with 2 perfectly balanced probes that flatline in A-B mode. ( no probe gnd clip using removed tipclip with pin&amp; exposed gnd probe barrels only )  Then for current meter, use 10M probe with shorted tip using gnd clip wire as a loop around cable and then stimulate potential impulse event.  Then measure diff. voltage at each end and measure amplitude and edge jitter for violations to safe operating area from CM input range.

Once you determine the source of the egress and ingress and measured it level exactly., the solution is easy.  If you can't wait for an event. Create one with arcing handdrill motors on a parallel extension cable. Discharge 100V in a &lt;1 microseconds on a loop wire from a charged ultralow 1uF ESR E-cap. That will safely give  1GV/s transient on a 1m antenna that will span the spectrum to a few hundred MHz

That's all I have to say for now.