yeah that is what I was going to say. There are multiple factors which can affect your connection and the fact that you are getting sporadic spikes means there is a utilisation issue somewhere like Igni has intimated. The ping monitor he wants you to setup tracks the pings over the internet to your shub (not pc) and will let us see if there is an issue with your internet connection or your network connection (lan) between the shub and pc. It will look something like this:
The biggest problem you have got, like jb has suggested, is that the shub is absolutely diabolical for wireless. I know you have probably been told it is the bees knees and seen the advert on tv for perfect at home multi user use but it isn't. If you are all going to use wireless you really need to buy a proper router which can sit next to the shub and the wireless on it will be far superior. We can recommend some good ones for £50-60 if you want to get your housemates to chip in. The powerline adapters can be a bit flakey as well because the connection/speed you get is entirely dependant on the quality of the electrical cabling in the house.
The best thing you can do to satisfy your curiosity is wait for a morning when everyone is out the house, disable wireless on the shub (so you know no dodgy downloads are going on) and run a patch cable from your pc to the shub so you have got a direct wired connection. Your connection to the shub will be faultless for all intents and purposes and any ping issues and reduced download speed will be to do with your wan connection on the VM side. If you are in a student area you might find your local node is massively congested and that is why you'll notice such huge swings in performance between morning (when nobody is home) and the evenings and weekends.