Hi Guys,
There are two sources of interference that can upset DRO's. Conducted and radiated, each has its own method of reduction. Sometimes both combine to cause problems that do not succumb to one cure or the other.
One way of reducing radiated interference, is by screening. That is putting a conductive barrier between the DRO and the source of the interference. Screened cables is one, putting the electronics in a conductive metal box could be another.
Conducted interference can be reduced by filtering. This could be as simple as wrapping the power lead through a ferrite ring, or using a ferrite clamp on the cable. Or as complex as inserting filtering devices in line with the actual power cable.
There are a number of techniques that can be used to help determine what type of interference is causing the problem. For instance running from batteries, or putting the electronics in an earthed biscuit tin.
In one case that I know about, a simple modification to add a capacitor across the power supply inside the display solved the issue, another required the replacement of the motor brushes.