Funny you brought this up now, I tried ferrite cores on the RFX at 3 seperate houses last week where we had RF issues with MRF-300s. In all 3 cases it solved the problem.

The core was 1" long, 1" diameter, and 1/2" hole through the middle. It was a split core so I could wrap the antenna wire through it 12 times. I do not know what type of ferrite it was, I've read "43" is the best material for around 400Mhz.

All 3 locations had severe enough RF that the RFX-150 LED would flicker and/or light fairly solid. After 12 wraps through the core, not a hint of interferance and the reception was exceptional.

Before I tried the ferrite I re-routed the wire and antenna to every possible location and unplugged the emitters but the light was still flickering.

The ferrite core had no effect at the MRF end, but placed within a few inches of the RFX it eliminated the interferance completely.