Minecraft – stop the FTB launcher from refocusing while it loads the pack

If you attach a logistic chassis with an extractor module (either mark 1 or 2) to the side of the furnace it will extract any excess products from the furnace and send them to the default route, do not worry about the extractor module taking items that the crafting pipe was waiting for as it has a lower priority so will not take the items unless no other pipe wants it.

According to this Feed the Beast forum, it is related to the loading and unloading of worlds. This applies to an issue Forge is having with the ChickenChunks mod because of the way it unloads worlds. Try going to your Advanced Java Options in the launcher (options --> Advanced options) and entering -XX:MaxPermSize=256m -XX:PermSize=128M in the Additional Java Perimeters field.

Linking to enough Mystcraft worlds and Twilight Forest multiple times can also be the cause of severe memory leaks. There also exists a semi-rare bug with causes FML (Forge ModLoader) to leak memory no matter what you do. Another common solution is to disable all of the wireless redstone mods, such as RedPower 2 and WRCBE. Basically, if you had made any Twilight Forest and Mystcraft worlds before, it is likely to happen.

Direwolf20 gives his two cents about the problem here: