The normal list of suspects. Computer/UPS power supplies, cell phone chargers, certain flat screen TVs, surge suppression strips. It may be necessary to look at smaller sections at a time. Display 1-10, 11-20, 21-30 or just a single record a few times to be sure the same results are seen each time. When a record is found that contains an address for which there is no device, update that record with one of the EZX10RF groups. When isolating sources of interference a technique that has worked at times is to plug the PLM (in this case both) into a good extension cord and move the PLM plug point away from the computer equipment. When it works reliably at times and not others what appliance is On or not On between the two times. Unfortunately there are no good signal analysis tools for Insteon. I use a Monterey Power Line Analyzer which is an X10 analyzer. It can detect general sources of noise when Insteon traffic is not flowing. The problem is that Insteon traffic looks like invalid X10 segments so it cannot be used effectively while Insteon messages are on the powerline.
EDIT: it might be helpful to understand that to pull a link database containing 50+ links more than 400 Insteon commands have to work back to back for that display to be successful. That level of sustained command activity will show powerline issues that will never be seen during simple device or scene control.