Since IR is using light, the biggest interferers are fluorescent lights and the Sun. Other types of equipment should only interfere if they are in close proximity to the USB-UIRT. If in doubt, move the USB-UIRt away from equipment as a test to see if things change.
Another thing you should try is to grab one of the factory remotes that came with your TV/stereo/etc. and see if you can get consistent codes with any of them. If not, I would be suspicious of a bad unit.
-Jon