Might have found the EPG Problem/Doh!
Sam,
Thanks for your suggestions; my cable run is brand new and is only about 15 feet long from the Dish 1000 to the back of the 622. I will check the connections out, but I MIGHT have found the problem: RTFM!
In the index "troubleshooting the program guide", the manual says that if all channels are not displayed, then take a look at the favorites settings and be certain that it is set to All Channels. In my case, it was set to HD Channels. Apparently a new program guide would download when I forced it, but sometime later the setting of HD Channels in the favorites section of my setup caused all of the non-OTA and non-HD channels to drop out from the guide
When the channels would disappear, I definitely had some non-HD OTA channels still showing up in the guide (such as channels 51 and 69 here in San Diego). I think the installer left the favorites set to "All HD" last week. Eventually, all of the satellite-delivered non-HD channels would disappear from the guide. I cannot explain why the direct tuning of the missing channels would usually not work, but I saw this several times myself, so it is not just a family training issue.
I will report back on whether this is a long term fix/correction of operator error.