Your best bet is to try removing the antenna from the MRF-100 and seeing if that helps. Going to a different MRF will not help because you would have to have it set on ID "0" to use it with the URC-300 and it would suffer from the same RF interference. Also, you should post any future questions about the URC-300 in the Universal Remote Control "Consumer" forum.
Mike
www.SurfRemoteControl.com