I've recently been playing Feed The Beast without modding vanilla minecraft. I've been using the FTB launcher which has the minecraft files already loaded in the Direwolf20 pack. For some reason it starts lagging after about three minutes after I start my map. I exit out, restart, and the same thing happens. I have added more ram to the Java runtime parameters. I also added more RAM from the FTB launcher options. I then tried installing optifine and using most of the fastest settings. All of this gave me almost no results. The debug screen says I have 600mb of alloted memory, but I set java runtime and the ftb launcher for 3GB. I have tried opening minecraft.jar with Java SE Binary, but it keeps saying "ERROR: Invalid or Corrupt jarfile." I am assuming this is because it is a modded .jar. If anoune knows a way around this please let me know.