Ongoing FPS drops in Summoner's Rift (new and old)

Hey there, I wanted to bring to Riots attention an ongoing issue I've been having with large FPS drops on a fairly high end system. Now then, this only seems to happen with VSYNC on, however, I still get random, sizable drops in FPS without VSYNC, but never below about 70 frames per second. With VSYNC enabled, I'll go from a solid 60 down to around 40. This lasts for a couple of seconds at the most, but a couple of seconds is all you need to die. Also, with old Summoner's Rift, it seems to occur mostly near the river. This also used to happen with my old HD6870 video card, but after a while, it was normal.

My system is as follows: AMD FX8350 16GB DDR3 1600 R9 270X 4GB Win7 pro x64

Also, since I'm here, I'd like to tack on a question. Why doesn't antialiasing seem to work on AMD video cards? I've played on my cousin's laptop with an nVidia chip and it works fine there, but with the option checked it doesn't work at all with my old HD6870 nor my 270X. Wassup with that, rito?