Problem:
When i check my internet connection on speedtest.net exactly when i test my upload speed the router is stopped for few minutes and is restarting.
Factory reset or last firmware - no result.
The solution for me was a flashing to the version of My_Net_N900_1.04.11
Link http://download.wdc.com/nas/My_Net_N900_1.04.11.zip
No more upload problem.