punter16, I tried what you suggested.
1. I saw no difference in performance after moving to several different locations, far away from other equipment.
2. We use an MX-980 for a different television in the house. I copied the file for the broken MX-980 to this remote. Then, on the new MX-990, I made a simple file with only working volume up, volume down, and mute buttons. I set both of them to the same RF ID. The MX-980 operated the equipment perfectly. The MX-990 continued to have the same issues.

Duct Tape, here's a link to the file I made for the #2 test above.
[Link: 1drv.ms]

Because the MX-980 works so well and the MX-990 does not, I'm starting to wonder if it's a problem with the MX-990 in general or with this particular MX-990.