Hi Scott,
UPB is a very reliable protocol, but sometimes the right device can totally kill things.
First, which brand switches are you using? And what sort of repeater are you using? Do you have a single breaker panel, or do you have any sub-panels? Are any of the switches off a sub-panel?
I had great signal and reliability with my UPB setup at my last house that was all CFL, had old appliances (100% electric) and had a pool pump... at this house, everything was great until I moved the PIM to where I really wanted it - in the wiring closet.
Simply Automated tech support was great - they showed me some tricks for toubleshooting using Upstart (for starters, do a Comm test and see what you can and can't talk to). They even reviewed the file and suggested a few things. Their next suggestion was accurate but a little tough - they basically asked me to unplug everything in the house and test - then slowly plug things back in and keep testing.
Well - I did that and things never got that great! So one day I screwed with moving breakers around, swapping phases on some circuits, etc - and for a minute made things worse - so I put everything back exactly the way it was - did another test - same as when I started. I was basically giving up, but while I was in the wiring closet, I decided to move a few things around. Without really realizing it, I plugged my DirecTV power injector, the UPS and one other thing into a different power strip (a high end strip that does filtering apparently - I didn't even realize). When I did one more network test - all of a sudden my switches that were unavailable before were now showing 7-13. I had never unplugged those things because the wife/kid were watching TV to stay busy while I tinkered.
During this whole thing I never had any noise except after screwing with the breaker panel and not getting a solid connection on one of the breakers - when I put things back I never saw noise again.
My advice - move the PIM around - see if that helps... in my case I had it plugged into the same source as my noise generator - talk about bad luck! If not, start going around the house and either unplugging things or putting in X10 noise filters and testing things out (just a couple filters - filter out whole power strips or turn things off long enough to test).
