I suspect you are not getting that data rate all the way from your provider connection to whatever is your Netflix box. For the past five years, I've not have had Netflix fail to keep up with my connection (without the service totally shutting down). If it goes into "buffering" the bottleneck is someplace between you and the network server.
Bottlenecks can be anywhere, and all I can offer will be a few examples that might suggest what to look for locally.
I've done Netflix fine with a solid reliable 3Mbps cable connection, feeding through my 802.11b/g/n- n300 WiFi router/switch to a wired connection into a BluRay player, a wired connection to a computer, a WiFi connection to a computer, and a WiFi connection to a ChromeCast device. All the wired connections are running at least 10 Mbps, but not over 100 (limit of router). WiFi devices, all are at least 802.11g. NF has been running on "automatic" which means they measure my bottlenecks and don't try to send more than I can handle.
Cable co upgraded my plan to 50 Mbps, but fastest my modem can handle is in the 12-17 Mbps, but that was enough for NetFlix to run me up to full HD, and send me 480GB of data the first moth after upgrade. None of my downstream bandwidths increased, and by then all my watching moved to The ChromeCast, i.e. 802.11g or n WiFi speeds (54 Mbps max, 20 Mbps typical). FWIW, my WiFi router is within 20 feet of the devices using it, no obstacles.
OTOH, my kids have been on DSL, everything through WiFi, and they have a lot of buffering problems. One possibility, ATT is not delivering on the DSL. Another, WiFi traffic or physical obstacles are slowing the WiFi connection. A single 802.11b device will slow traffic to that standard, if the access point is single channel, so nobody runs faster than 11 Mbps, and everybody is competing for a share of that. BluRay player, one or two laptops, a tablet or two.
Another, at my late brother's house, connection was through cable rated 10 Mbps, feeding single channel 802.11b/g/n WiFi. Router/access point on 2nd floor, all users 1st floor or basement. If we had one video streaming device, a couple of smart phones tapping WiFi, one or two laptops running, there would be no problems streaming NetFlix. But get all the siblings, nieces and nephews there connecting there smart phones, tablets, PSPs et al, somewhere between 10 and 30 devices the WiFi network would get saturated and nobody could do anything. The router would even start dropping connections.
which makes me wonder. Is your ISP actually delivering the bandwidth advertised (e.g. mine is delivering about 1/5 of what I'm paying for)? Is your local network letting that bandwidth through (especially if iusing WiFi where a single device can slow it all down)? If you are using WiFi, do you have so many devices tapping your connection that not enough is left for streaming video?
