DVR stops playback at certain times

Hello, for the last week or so, while playing back a recorded program, at 6 PM the playback stops and after a second or two, the TV will show whatever the current channel is set for. The same will happen at 7 PM while playing back another recording. I've experimented with playbacks at earlier times of the day, but do not see this symptom at those times. I can't say it's only at 6 or 7 PM, but it is happening every day at those times.

I thought it might be related to the DVR simultaneously recording while another recording is being played back, and when the recording ends, say at 6 PM, it also stops the playback, but experiments show that not to be the case.

I've contacted COX service twice. The first time they updated the DVR firmware. That did not solve the problem. The second time they replaced the DVR box. That also did not solve the problem. It smells like a firmware problem, but I'm not sure where to go from here. The DVR is a Scientific Atlanta 8240 HDC.

Thanks,

TIm