I have no new devices in the house, something is interfering with the signal from the CM15!
Any thoughts on how to start isolating the source?
Think real hard about that first statement. Are you sure nothing new? Have you moved anything from one outlet to a different one? etc.
As far as troubleshooting: a X10 signal level meter is invaluable, but is a pricey investment. Without a signal meter you can try the following, it has always worked for me when moving into a new home and having X10 problems.
1. First turn off all CFL bulbs and check if X10 begins working again. If so, you have an aging CFL that has become noisy.
2. The general idea: Place a TM751 or CM15A at one end of your house (preferably close to the electrical panel) and an Appliance Module with radio at the other end of the house. Turn OFF all breakers except those two breakers feeding the CM15A and the AM. Use a Palm Pad to send ON-OFF commands to the radio as you turn on one breaker at a time while listening for the appropriate action from the radio. Turn the breaker back off and move on to the next. Continue this until the radio stops responding to the ON-OFF signals. That breaker/circuit will contain the offending noisy device.
If the radio continues to respond and you have cycled all the remaining breakers, then repeat this process, but this time leave each breaker on as you move on to the next. When the radio quits that circuit will have a noise maker, but it is not likely to be the only one noise maker you have.
You can modify this process, for example you can probably leave you CM15 in it's present outlet.