Is it possible that your recorder is rebooting? I was just looking for some information on it... My NVR used to reboot itself randomly a few times a day, but now that I've updated it's firmware it's rebooting around about every hour.
I've noticed that with the Hiks I have connected to a dahua NVR using SmartPss for playback. Funny thing is when I went to play back the same footage on an older version of PSS there were no gaps. There were also no gaps when using playback via browser interface.
This is your issue you're using a PC hard drive to do the the work of a server.
Data is lost during high usage, depending on how many camera's you have as the PC hard drives can't keep up. Even good drives suffer you will notice pixelation on play back due to lost data and what your experiencing gaps.
Seagate drives seem to be the worst for this.
ive got same problem, its not nvr rebooting because the gaps are at different times, other day 1 camera seems to of had a gap of over an hour :S when other cameras were recording fine.
Usng a Hikvision 7608 E2 NVR with latest firmware, an 2x WD 2TB Purple NVR Drives
I did manage to sort this out by installing freenas on on my media server and use it for my cameras and storage only I think the problem might of actually been network activity or what whoslooking mentioned I will be able to diagnoses it better with freenas but so far its playing pretty good not one gap and only around 30 watts load I am pretty happy with it.