Several clients in our market [including my wife] are complaining that the TWC channel guide is often "not available" on their Cisco DVR's, especially the black face 8640HDC. The only fix to restore the channel guide is to reboot the box. One client's DVR events are also not recording when the guide goes missing...

Has anyone dealt with TWC to correct this problem? Any ideas as to the cause?

I don't have an answer for you, sorry.
I will say this thought; out of all the service calls, this would be the easiest one to deal with since it has nothing to do with us, installation, control, etc.

I have the same problem with my box (Cisco8640). My SP is Rogers (in Canada) and it happens when a firmware is pushed to the box. The box doesn't reboot properly after the update and doesn't download the guide, thus cannot record any shows. Only fix I have found is like you said, a reboot of the box.

I haven't had the time to sit on hold waiting to talk to someone, then when I finally get to talk to a person be told that a tech has to come out to troubleshoot, when I know that its a hardware problem.

If rebooting solves the issue, have you considered putting a blue bolt device on it and do a scheduled reboot once a week? I don't have the technology at my house, so obviously I don't understand implications that might be associated with my suggestion.

The issue with this is that the providers can push a firmware anytime. If they push the firmware on a Tuesday and your system is scheduled to reboot on Sunday then its a useless reboot.

You could have the system push a reboot every night but then you have to know what time the providers send their updates to ensure that you aren't rebooting before or during the update.

Blue Bolt would solve the problem that the customer doesn't physically pull the power, but they won't know they have to perform a reboot until they go and watch TV, at which point the box may have already missed recording something.

excellent points.

I know this sounds crazy, and it's very much outdated...

I had the identical issue with my TWC DVR several years ago. I'd alternate hitting the info, guide, exit etc a few times. 90% of the time the guide info would refresh within a few hits of this series.

The issue self corrected in the long term so I only needed to use this method for a few months.