The color of the HEALTH BARS ARE GREY (not red for ennemis or green for allies). So, in teamfights, it is really hard to focus an ennemy because every health bars are grey!!! I tryed to uninstall and reinstall LoL but nothing changed. Help !?!!
Stop dying. 
The only time i've seen this was my own health bar when the game didn't load properly. Have you tried restoring defaults for video? (Then changing it back to your preferences)
simply control+esc and then re-enter league, thats what fixes it for me
I had the same problem, you're probably playing in a spot where the internet is bad, try going closer to the modem
