" It consistently jumbles the inputs so badly that it takes three or four attempts to input a channel correctly, or to navigate to an item within the guide (after selecting the wrong item a few times)"

So it's not the "input" or "source" that's the issue, it's the channel selection from your AT&T cable box when you manually press the buttons to find a channel?

"This affects only the keys on the remote itself and not the macros, which function flawlessly. The macros were programmed by the installers and the system functions just well enough for them to tell me that the input issues are "normal." Somehow I don't believe that to be true, so I need a little help!"

Understand that the macros were derived from the commands on the remote, so if the macros work flawlessly, then in turn the buttons should as well. Sometimes when you select channel buttons there is a delay needed for the cable box to respond to each numeric selection. When selecting them manually, you may be pushing the buttons too quickly and the cable box can't keep up- slow down your manual channel button pushing. In addition, some boxes may respond better with an "enter" or "select" command at the end of the numeric button press sequence. When you mention the macro's working flawlessly, are you referring to the "Favorite Channels" that the MX-450 features?