Thanks for all the suggestions! I figured out the final (I hope) issue: Some of the keys I was having trouble with apparently had punch-throughs that needed to be cleared out. Once I did that, everything seemed to work pretty consistently.

This remote really, really needs a simple "Clear Everything (and I mean EVERYTHING)" command. I don't care if it wants to ask me "Are you SURE?" 16 times in a row to make sure people don't accidentally mess up their remote - I wasted a TON of time because I thought the remote was "clean" and it wasn't....

Again, I really appreciate the support. I'll try to get this added to some MX-500 FAQs...

-Fred