PS: I have just noticed at the top of the Networx graph photo you posted that it says 'All Wireless Connections'. Speed fluctuations like that can also be a wifi issue, so before anything I would try to use ethernet to rule out the wifi. If it works OK on ethernet you can save a lot of hassle tweaking and work on that (change channel, change channel widths, change PHY). If it still happens then you know it's probably a machine/disk/cache issue.