Offline gaming problem?

I have a very bad lag in my games. I know this isnt because it applies to offline games. Games like Minecraft beta(singleplayer) and other offline games are the one that this lag is happening in. It starts out fine and 3-7 mins later BOOM MegaLag. I mean the kinda lag that is like wayyy less frames per second.once my friend suggested game boaster, but that didnt do anything.Please help me i even reformated my whole computer. I am using Windows 7 64bit. if you need anymore info ill be glad to respond.Thank you for your time.