I've seen DTCs freak on power failures for no other apparent reason.
 Swapping DTCs seems to fix the problem along with placing it on a surge
protector.  Sometimes the "BAD" DTC has gone back to normal after sitting
with the power off for a week or two.

The only other thing I can think of is after all, it is connected
serially with umpteen other devices.  With a guess that not all of them
are grounded properly.  I've noticed that the practice of grounding a
3-wire RS-232 to frame ground on pin 1 has declined over the past two
decades and that may exacerbate the problem.

Good Luck.  Insha'llah.