I have an MRF-350/RFX-250 with the MX-980. I've been trying to figure out which item is defective and needs to be replaced as it seems to work intermittently. I've unplugged the power to the MRF-350 to "reboot" it and thought things were working better but recently noticed a time when pressing the remote buttons didn't cause the status light on the MRF-350 to light. I've also turned the MRF-350 to ID#0 and don't see any blinking of the status light on 250 or 350 without any button pressed.
Any tips on how to determine which item is defective?

Definitely take it off ID 0.

Intermittent performance issues are often caused by RF interference or problematic placement of the receiver.

Have you tried a different channel other than 0 and the one you were using before? The problem with RF interference is that it can be sporadic. Everything is working fine when you are testing it but when you sit down to use the remote again you may have issues.

Everything is working fine when you are testing it but when you sit down to use the remote again you may have issues.

I think that summarizes my issue. I'm currently on Channel #3 and used 0 to check for interference and put it back to 3 when finished. The cabinet I have the MRF 350 and RFX 250 in, also has my 8 port switch on the same shelf.

Sometimes you get lucky and see the activity light glowing all the time, or fading in and out intermittently. Sometimes you just won't catch it. Some devices may only put out interference when they are receiving commands. It can be random.

The bottom line is, move the antenna to a different spot. Some systems where I am getting interference problems, where I may be using the remote only 15 or 20 feet away from the antenna, I just fold the antenna down all the way to reduce interference. In days gone by, some removed the antenna, as the remote puts out plenty of RF for close in use.

Move that RFX around and retry.

I've moved the RFX 250 to a different location. Everything works but when the system 1st turns on when cold, the cable box initially doesn't get channel change input. After a few minutes it works fine. Tried to use the STB remote (IR) and it seemed to work.

Just wanted to update everyone on this problem. It turns out that the MRF-350 was bad. I replaced it and now everything works perfectly as it did before. Thanks dalto and tomciara.