It's possible that your problems are due to line noise that causes the logic in the devices to not work properly. This is different than line noise that blocks communications between properly working devices. The symptoms manifested by these two different types of problems are quite similar. It's important to determine which type of problem you have since the methods to fix one type of problem are ineffective at fixing the other.
A simple test to determine if you have the former problem is to direct the PLM to report it's links table. In the admin console Tools->Diagnostics->Show PLM Links Table. By selecting Start the contents of the PLM's links table will be presented. It's the number of links reported that is important. Do this 4 or 5 times and see if the number of links reported varies. If it does then you probably have line noise that causes the devices to not work properly (i.e. not communication issues). Note, the Count button on the dialog box won't help - it just reports the number of links. It's the act of getting the PLM to retrieve and list the link table that is important.
If you have this type of problem it's relatively easy to locate the source. My strategy was to first get a minimal working configuration, i.e. where the number of links reported is consistent. Then I planned to add parts of the network in small increments and testing (via listing the links table) after each increment. In the first step I had only one breaker on that ran the PLM (and the ISY connected to it), the router, and the PC. Testing this showed that I had the problem even in this minimal configuration. Replacing the PC with a laptop fixed the problem; i.e. listing the links table worked consistently. I was lucky that I found the problem in the first step. But, the point is that you can try different things and determine quickly and reliably if you've made a difference.
I have a theory about why this PC caused the problem. In a sense, it's not really relevant, since the fix was to either replace the PC, or it's power supply. Two Filterlincs wired in series with the PC didn't fix the problem. If you're interested you might want to check out http://forum.universal-devices.com/viewtopic.php?t=1188