This is definitely not a problem of the Oppo. I am playing m2ts files via 802.11n 2,4 or 5 GHz and both works fine. Sometimes I have problems with the WiFi link when somebody in the neighborhood is using similar channels as I do or in 5 GHz band there can be some commercial applications disturbing - at least in Germany.
However, you should try to connect a portable disk directly to the Oppo to check if it is the connection or the server. At least this would be my recommendation.
I can assure you, that the Oppo has no problems with high bit-rate files. Look at the WD-TV, this is a small box for under 100$ and it can also do this.