Assuming you are using a Mac Pro and not a MacBook Pro (some people get these forums mixed up) the problem is likely not with your computer.
I get slow internet periodically and my computer has never been the problem (I've got a 2008 Mac Pro running Mavericks).
Usually it's my router. My solution is to turn off the router, and the bridge device (which converts the radio signals to router-friendly) wait a couple minutes, then turn it back on. This resets the connection to my ISP.
My situation is a bit unique, as I get my internet signal from a Line of Sight radio transmitter a couple miles away, and sometimes trees or weather can affect the signal, but I usually start from the assumption that it's my router, and most of the time it is.
In the rare occasions when power-cycling the router doesn't work, a call to my ISP will often solve the problem, as in "yeah, we're having some issues here" or "we're repairing the transmitter" or whatever.
