What ISP Modem Mfr. and model # do you have?
Can you connect a wired LAN PC to the router and put some source files, 500Mb to 4Gb in size, on this PC and share out the folder. Then connect your wireless test PC to the router and do some Read and Write operations to the wired LAN PC and see if you notice the same thing...
Then change the WiFi settings from default to Single mode N, WPA2 and AES only on channel 13, disable HT 20/40Mhz co-existence if you have this feature under Advanced/Adv Wireless, using the same configuration as above...