I am having similar issue after i updated the patch 6.10. My Ping stays intact but FPS drops drastically from 100+ to 2-10 after every 2-3 minutes and stays like that for 1--2 minutes and this keeps on happening repeatedly, atleast 4-5 times in a normal 25-30 mins game. Literally the game is unplayable for me, i even tried reinstalling but no improvement observed.
Things that i have tried without any results:
-Defragment LOL folder
-Reinstall LOL client
-Force repair LOL installation files
-Uninstall my antivirus software
-close down all other apps while running LOL
I am at a loss as to what to do next. I even got myself a 5 match low priority queue
just becoz the game is damn unplayable. I believe this is something to do with the new patch and not my PC or internet as they were working fine few days ago. I hope someone finds a solution to this problem or riot releases a new patch to sort this out.