I certainly don't want to have to reinstall FSX or disable UAC in Win 8. Any other ideas on how to fix the missing letters issue?!
XPOI doesn't have any missing letter issue that should be fixed. If it happens, that's not an XPOI issue, but it's XPOI being the victim of some other problem that lies elsewhere. It's just that you will notice missing letters more than, for example, an AI airplane not being created, or a weather command not being received or sent, but if you are having Simconnect communication problems, they are happening with every FSX add-on that use it.
If it's not a permission problem, which might be cured by turning off UAC (which is something many FSX users do on Windows 8 anyway), it might also be caused by another add-on that is generating too many Simconnects commands in a short period of time, not letting any chance for XPOI to talk with it. Simconnect is a communication channel, and so it might be clobbered by any add-on that tries to "own it".
Again, this might be dependent on system settings too, because Simconnect uses named pipes, which are handled by the OS.