Hi, I have the same problem with a 715 DVR from Avtech.
The recording and motion detection carries on working but the network side just stops responding, It looks like a TCP stack problem with the recorders software, only a power off/on will cure it.
What AVTECH email address have you managed to get a reply on. I have sent numerous requests to the email addresses on thie AvtechTW website but dont get any replies. The distributor I bought from cant offer tech support, just to replace the unit, and I know that this will not cure it.
Thanks, Bill