After working fine for about a month, I got a complaint that the channel up down buttons didn't work any more. When I checked things out, I found that the MX-3000/MRF-250 combination worked fined with every component EXCEPT the satellite unit. But the satellite unit's remote would work just fine.

So I cleared out all of library IR codes and learned them all instead. No good.
I flushed the unit and downloaded everything again. No good.
I reset the satellite unit. No good.

Then I saw a couple of miscelleneous threads about IR overload with some satellite units. I tried the suggested trick of covering up most of the satellite's IR lens with electrical tape, leaving only a small sliver of space left for the 250's IR emitter. Everything is now working like a charm.

Thank you to others in the various forums for the suggestion! I still can't quite believe that a 1" piece of tape is the fix...