Cannot change FPS maximum?

So, I'll admit from the get go I do not have a very good computer. It's been fine with league using medium-low settings, and keeping the fps maxed at 30. However, recently my settings recently changed to max 60, and now I cannot reduce back down to 30. If I go into the settings and set the drop down to 30, it freezes my screen a moment, but stays at 60 when it catches up, and still reads as maximum thirty when I go back into the menu. This is causing severe frame drops and occasionally putting my computer into sleep during games due to the heat. Any ideas/reason this is happening or how to fix it?