You need to determine if the lag is due to your PC or the network itself. Try running multiping in the background to see if the latency is actually jumping when you experience lag. A lot of times people think their Internet connection is flaking out, but it's really the PC itself slowing down.
You could also have a look at task manager to see what kind of background processes you have running. If the lag is pretty consistent every five minutes, then it could be some service you have running on your computer that's performing some action every X minutes, and thus slowing things down.