Please Help, during champ select my client freezes and so does my computer

At first the client begins to be spotted with pixels(white, black, and red ones), next the champion select screen completely freezes and so does my computer. Sometimes it unfreezes but most of the time it doesn't, I lose lp and time because of this.I did not do anything before the bug happened it just started randomly. My mac is fairly new(2016), using os sierra... I even deleted the entire game and then downloaded it again, but the bug persisted.