[MC-55996] Server: Regular freezing, chunk

Game freezes for at least 10 seconds, about 3 times in a row, then resumes ok for 5 minutes or so. Only happens in the eastern region of my base. The Shift_F3 screen says Chunk_Loader is chewing all the CPU (and Task manager shows massive CPU usage). It throws an error message, given below, but keeps on chugging after a frustrating delay of 0 FPS. Maybe a bad block somewhere? If so, how can I locate it? Happens both with and without Optifine. I haven't used summon commands or used 3rd party editors to add weird blocks.

Exception in thread "AWT-EventQueue-0" java.lang.InternalError: HTHEME is null
        at sun.awt.windows.ThemeReader.paintBackground(Native Method)
        at sun.awt.windows.ThemeReader.paintBackground(Unknown Source)
        at com.sun.java.swing.plaf.windows.XPStyle$SkinPainter.paintToImage(Unknown Source)
        at sun.swing.CachedPainter.paint0(Unknown Source)
        at sun.swing.CachedPainter.paint(Unknown Source)
        at com.sun.java.swing.plaf.windows.XPStyle$Skin.paintSkinRaw(Unknown Source)
        at com.sun.java.swing.plaf.windows.AnimationController.paintSkin(Unknown Source)
        at com.sun.java.swing.plaf.windows.XPStyle$Skin.paintSkin(Unknown Source)
        at com.sun.java.swing.plaf.windows.XPStyle$Skin.paintSkin(Unknown Source)
        at com.sun.java.swing.plaf.windows.WindowsScrollBarUI.paintTrack(Unknown Source)
        at javax.swing.plaf.basic.BasicScrollBarUI.paint(Unknown Source)
        at javax.swing.plaf.ComponentUI.update(Unknown Source)
        at javax.swing.JComponent.paintComponent(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at javax.swing.JLayeredPane.paint(Unknown Source)
        at javax.swing.JComponent.paintChildren(Unknown Source)
        at javax.swing.JComponent.paintToOffscreen(Unknown Source)
        at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)
        at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)
        at javax.swing.RepaintManager.paint(Unknown Source)
        at javax.swing.JComponent.paint(Unknown Source)
        at java.awt.GraphicsCallback$PaintCallback.run(Unknown Source)
        at sun.awt.SunGraphicsCallback.runOneComponent(Unknown Source)
        at sun.awt.SunGraphicsCallback.runComponents(Unknown Source)
        at java.awt.Container.paint(Unknown Source)
        at java.awt.Window.paint(Unknown Source)
        at javax.swing.RepaintManager$3.run(Unknown Source)
        at javax.swing.RepaintManager$3.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
        at javax.swing.RepaintManager.access$1100(Unknown Source)
        at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
        at java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
        at java.awt.EventQueue.access$200(Unknown Source)
        at java.awt.EventQueue$3.run(Unknown Source)
        at java.awt.EventQueue$3.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
        at java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.awt.EventDispatchThread.run(Unknown Source)

Server: Regular freezing, chunk_loader chewing all CPU