mine did this too. WNDR3400v1. In fact, this router is warranted forever, and they have sent me a couple. However on the last go-around, they said
redownload the current bios from the website
reflash the bios
do not reload any saved configurations. do them de-novo (i have a lot of custom features)
There were a couple other things.
use inssider
www.inssider.com
to identify competition on the 2.5 and 5 ghz network channels and move to less used channels. they used to have a free version; i dont know if it is still free
They told me to disable SIP ALG on NAT Filtering (leave open). Quite frankly i didnt see what this had to do with anything.
I am pretty sure that the rest of my setting are default.
I have not had a problem since - and this was several months ago. i am running V1.0.0.38_1.0.61
2.4 GHz radio is channel 11
5 GHz radio is channel 44