Aside from the timer, what other controllers/remotes do you have? When I had a similar problem years ago, it turned out to be a Mini Controller that had gone bad. It's probably not a noise problem as that would usually block signals rather than cause false signals. It's also unlikely to be a neighbor's controller since not many neighbors will be using X10 today, although it is possible.
What timer are you using? To eliminate the timer as the culprit, you could leave it unplugged for 24 hours and see if there's still a problem.
If there re other controllers/transceivers do the same - one at a time - to find the problem.
Lastly, what new things have you plugged in recently? Chargers, LED lamps, etc. While these won't usually cause false signals, they're worth looking at. Also remember that a "wall wart" may not cause problems for years and all of a sudden cause problems while still working.