Jerky lag on Minecraft OS X

I've been playing minecraft for a year now, and have had this weird problem where about 5 minutes after I start playing, the game gets jerky (will lag for a second, then be normal for 2 seconds) and after 5-10 minutes, it stops happening. I don't get why, this is my video settings It happens on any texture pack. My macbook is a late-2009 model, 2.2 ghz, dual core, 4gb of ram.Anyone know why this happens?