I can't tell you what in your house may be causing interference
. I can only tell you that you can test this by unplugging EVERYTHING in a room then testing, then the next room and testing and continue this until you hopefully find the culprit. If none of that works then it's to the fuse box, circuit by circuit until you find it. Or you could order a new Z-Stick for $50 and try that first (which is what I did) and then all bases are covered. I didn't need the Z-Stick because it ended up being interference.
Your mileage may vary.