When a heavyweight tooltip pops up under the cursor, the tooltip flickers repeatedly.  No problem with lightweight tooltips.

The following code will demonstrate.  move the mouse into the bottom part of the frame that appears so that the tooltip will popup underneath it.

import javax.swing.*;

public class ToolTipFlicker {

    public static void main(String [] args) {
	JFrame frame = new JFrame();
	frame.setBounds(20, 20, 100, 100);
	JPanel toolTipPanel = new JPanel();
	// force a heavyweight tooltip using long text
	toolTipPanel.setToolTipText("This test text is wider than the window");
	frame.getContentPane().add(toolTipPanel, java.awt.BorderLayout.CENTER);
	frame.show();
    }
}