DVR/Cable box is lagging and freezing up

My DVR/Cable box is lagging and freezing up, especially in the evenings and night.  When I press the Guide button and begin to scroll, it will freeze up for several seconds and I have to press Exit repeatedly to get it out of the guide.  The same happens somtimes when I type in a channel to go to directly.  The box will freeze up, but the TV show I'm watching does not freeze.  Also sometimes when I am watching a recorded show and hit fast forward, or even rewind, it will keep forwarding after I press Play over and over again.  I have to hit a number of buttons to get it to stop fast forwarding, then I have to rewind back to where I wanted it to play.  Many times when I am trying to get the box to respond to commands, it will say "One Moment Please..." or other times I get a window that says "We are unable to connect to your cable box."

At first I thought it might be a problem with the box, so I took it to the Cox store and swapped it out for a new one, but even the new one has this same problem, and that tells me it's not in the equipment.  I had a technician come to the house around lunchtime, but we couldn't duplicate the problem.  But as mentioned, it happens at "peak times" like in the evenings when I assume a lot of people are watching TV.  

I have called tech support many times, and I have done several resets to the box either by requesting a signal to reset, telling the box to reset in the Help menu, and by unplugging the power cable from the back of the box.  These don't seem to help.  When I've talked to tech support, they tell me that they show my equipment as working properly.

Any ideas what could be causing this lag and freezing up?