Consider trying a different DNS server. (Google 126.96.36.199, 188.8.131.52 or OpenDNS 184.108.40.206, 220.127.116.11)
It is possible your router is not up to the task. If you still have your original router, it may not be able to run at 50Mbs (routers do not have the fastest processors in them, and if this is an older unit, it may not have imagined 50Mbs). Also 50Mbs is faster than 802.11g speeds. Are you using 5GHz 802.11n for your router and Mac?
It is possible your WIFi is experiencing extreme interference from neighbors.
When you switched your service, what equipment was changed? What stayed the same in your network?
Along with the excellent information from BobHarris, another one occurred to me.
If you're on DSL, a bad or weak telephone line connection can cause a lot of speed fluctuation. I had that problem here when I moved to higher speeds (when they were finally available in my area). The phone line was daisy chained throughout the house, like most are. All that bouncing from one plug to another can cause a lot of interference and signal decay. The phone company came out and ran what they call a "home run" line. Which is a separate line from the inlet box on the outside of your home to a dedicated outlet in the house. No more problems.
Related to that (again, DSL), is the wire running from the wall outlet to the DSL modem. Keep that short, like less than 8 feet and use a high quality phone cable.