I have an odd issue, if I upload a file to my NAS on my lan or to a say a web server on the internet I see upload speeds fall to 0 every so often, at least a couple of times and then it may stabilize, but if anyone walks past the router it will drop to 0 again. This only happens with uploads, downloads are fine and only on wifi, if I connect via Ethernet it is fine. 

I just replaced another brand modem/router which had no problem like this.  Modem is in line of sight only about 4 meters away from the PC