That sounds like Packet Loss. Unfortunately, if that is the case, there's not much you can do, but in order to test it, run a tracert through your pc's command prompt, and set the destination as Riot's server. I apologise, but I'm on my phone now and can't remember the IP for them, but a bit of digging on these forums will show you exactly what you need to do.
Packet Loss isn't like normal lag, because instead of a simple slow flow of data, the data is actually not being sent at all. It may start at 100%, but by the time it gets to your computer, only 70% may have actually gotten there. Packet Loss is more frequent when the connection from you to the host is longer, therefore requiring more 'hops' where your data needs to pass through a checkpoint or another host. The fewer the hops, the faster the connection, and fewer risks of Packet Loss. The tracert will show you if packets are being being lost during the time it takes data to go from Riot's server to your computer, and specifically how many hops it takes to get to you and how much data is being lost.