I have a 1.5.2 instance of minecraft with 79 mods, and after about 15 minutes of being in-world, it starts lagging regularly, then after 3 or 4 increasingly severe freezes, it crashes.  It is rare to have any crash at all, but I have a crash log from MultiMC.  Here is the first part,  with a mod list and the entire log of what went on before the crash: http://pastebin.com/v8nVLSqp.  Here is the error that actually caused the crash, for convenience:

2013-05-21 21:24:51 [iNFO] [sTDERR] java.lang.NoClassDefFoundError: net/minecraft/world/CallableLvl2
2013-05-21 21:24:51 [iNFO] [sTDERR]     at net.minecraft.world.World.func_72914_a(World.java:4398)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:657)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:571)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:171)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:469)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)
2013-05-21 21:24:51 [iNFO] [sTDERR] Caused by: java.lang.ClassNotFoundException: net.minecraft.world.CallableLvl2
2013-05-21 21:24:51 [iNFO] [sTDERR]     at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:238)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at java.lang.ClassLoader.loadClass(Unknown Source)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at java.lang.ClassLoader.loadClass(Unknown Source)
2013-05-21 21:24:51 [iNFO] [sTDERR]     ... 6 more
2013-05-21 21:24:51 [iNFO] [sTDERR] Caused by: java.lang.OutOfMemoryError: PermGen space
2013-05-21 21:24:51 [iNFO] [sTDERR]     at java.lang.ClassLoader.defineClass1(Native Method)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at java.lang.ClassLoader.defineClass(Unknown Source)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at java.security.SecureClassLoader.defineClass(Unknown Source)
2013-05-21 21:24:51 [iNFO] [sTDERR]     at cpw.mods.fml.relauncher.RelaunchClassLoader.findClass(RelaunchClassLoader.java:227)
2013-05-21 21:24:51 [iNFO] [sTDERR]     ... 8 more
2013-05-21 21:24:53 [iNFO] [Minecraft-Server] Stopping server
2013-05-21 21:24:53 [iNFO] [Minecraft-Server] Saving players
2013-05-21 21:24:53 [iNFO] [sTDOUT] Unloading Player: <username>
2013-05-21 21:24:53 [iNFO] [Minecraft-Server] Saving worlds
2013-05-21 21:24:53 [iNFO] [Minecraft-Server] Saving chunks for level 'Test'/Overworld
2013-05-21 21:25:22 [iNFO] [Minecraft-Server] Saving chunks for level 'Test'/Nether
2013-05-21 21:25:22 [iNFO] [ForgeModLoader] [Thaumcraft] Removed 24 aura nodes when world 0 unloaded.
2013-05-21 21:25:23 [iNFO] [ForgeModLoader] [Thaumcraft] Removed 0 aura nodes when world -1 unloaded.
2013-05-21 21:25:23 [iNFO] [ForgeModLoader] Unloading dimension 0
2013-05-21 21:25:23 [iNFO] [ForgeModLoader] Unloading dimension -1
2013-05-21 21:25:24 [sEVERE] [ForgeModLoader] Fatal errors were detected during the transition from SERVER_STARTED to SERVER_STOPPED. Loading cannot continue

If anyone could help me track down the problems, that would be fantastic.

 

EDIT: I have also noticed that right before it is about to crash, my computer starts to "freak out."  Usually, the processor runs at about 30% while playing minecraft, but when it crashes, it jumps to about 90%, until it finally gives up and stops.

http://www.minecraftforge.net/forum/index.php/topic,5442.msg29619.html#msg29619