Try what dave said then change your house codes.
Sometimes the device that is causing problems isn't generating too much noise, but instead is absorbing the X10 signal and making it weaker (or be both generating noise and absorbing the X10 signal). This is typically caused by a Noise Trap. Fortunately using a the proper type of noise filter will also resolve this problem, so for the purpose of this article we will lump Powerline Noise and Signal Absorption together as the same problem even though they have their differences. A power supply is needed to convert the 120VAC that is in your house to the low voltages needed by electronic equipment. Traditionally a transformer was used to do this since they are easy to build. Transformers are very friendly when it comes to X10. Not only do they not generate any powerline noise, but they are inductive in nature meaning they provide a small load on the X10 signal and help filter some of the noise generated by the device from flowing back onto the powerline.
Transformers have some drawbacks though. If the device needs a lot of power, the transformer needed is big, heavy and expensive. Also they are not very efficient (you will notice that a lot of power supplies will get warm and heat is the product of wasted electricity).
Because of these disadvantages, many devices use switching power supplies. They are smaller, lighter and more energy efficient. For small power supplies, there is little cost benefit for using switching power supplies, but larger switching power supplies are much cheaper when compared to a transformer. For this reason computers have used switching power supplies for a long time. With new environmental regulations, more and more devices will start to use switching power supplies.
From an X10 perspective, switching power supplies are not as good. Not only are they not inductive in nature, but they will either generate noise on the powerline or have filtering capacitors which suck up the X10 signal. Newer designs are better than some of the older ones and sometimes replacing the powersuply is a better option than using a noise filter (or do both).
Some devices that use switching power supplies include (but are not limited to): PCs, Flat Panel TVs and Compact Fluorescent Lights (CFLs).
INFO from
http://kbase.x10.com/wiki/Noise%2C_Noise_Filters_and_Automatic_Gain_Control