I'm playing on a 2010 Macbook Pro and I'm getting some pretty big FPS drops throughout the game. Reducing video settings to Very Low with no shadows improves the framerate but it's still a rollercoaster experience. I've tried different caps to no avail. On the Windows client on the same computer, I get 60 fps capped with all settings Very High except Medium shadows, and it stays pretty stable except maybe dropping to 50 in extreme cases. Both clients are at full resolution.

I have a Macbook Pro of about the same time frame and have noticed that on the Mac side the computer doesn't optimize the use of the hardware very well. When I let it "auto detect" settings in LoL I have the same up and down issue. However if I drop the resolution from 1400x900 to the next lowest widescreen which is 1200x800 I think the game doesn't look as pretty but this problem is completely resolved.

I tried that, I'll get anywhere between 110 and 30 fps, 1200x800 with everything as low as it goes. And the drops don't seem to depend entirely on screen activity either.

...

Does this happen every game or just that one time? If possible can you load the r3d log from that game located in:
/Applications/League of Legends PBE/Logs/Game Logs/

Can you check in Activity Monitor if anything on your computer is using large amounts of the CPU?

Can you also post the following system specs:
- System and exact System version, like this: Snow Leopard 10.6.8
- Mac Name and Generation, e.g.: MacBookPro6,2
- CPU, e.g.: Intel Core i5 2,4 Ghz
- GPU, e.g.: nVidia GeForce 330M GT 256MB
- RAM, e.g.: 4GB

You nailed the system specs except I'm on OSX 10.8.2. I'll try a game now to see about other activity and then I'll load an r3d log. Also, to clarify, I'm playing Custom matches on Proving Grounds.

So far it seems to keep stable at around 60, still on lowest settings. I'll try some other game types a little later

Shadows on most games tends to lead to frame rate issues the higher the setting is. Which I have turned on and off shadows in between matches doesn't affect my frame rate much. Also your frame rate settings affect it too I would try in a custom game (password protected) with various settings until you find one that runs smoothly. That way you can get your settings going without being burden in a co-op or pvp match if something goes wrong. My computer is several years old running at 30-40 fps with highest shadow setting and 75 fps on average with no shadows. If your experiencing this in fullscreen I would see about playing windowed which seems to improve frame rate for most people including myself.

You can also try resetting the SMC, which has worked for several people.

http://support.apple.com/kb/HT3964

The crazy thing is though that for the boomjie client it runs great (running the game on the macbook pro retina). On the PBE client the game consistently lags and drops fps. I put the setting the same on both clients and have vastly different gameplay qualities.

Even moving the mouse around is just a chore to try and cs right now.