Also, even if you are not showing any interference, set the adjustment on the RFX-150 antenna to the lowest setting (counter-clockwise) and see what happens. I just finished a house with 3 MRF-300s with RFX-150s and in every case we showed no interference, but turned the antenna gain to the lowest setting and operation went up to 100%

If you some distance from the MRF-300 while operating the remote, start with the antenna at the lowest setting and try it, then slowly turn the gain up and see how operation is affected chances are this will solve it.

Also watch the RF light on the antenna too, I have seen these glow slightly yet the MRF-300 would show no interference and turning down the antenna gain fixed it.