What would cause large jumps in brightness during a solar eclipse?
Eric Pauer, who produced the chart, has an interesting web site that covers this eclipse.  It is evident that the weather was not great at the time of the eclipse, with intermittent cloud, and it will have been passing clouds that caused the variations in intensity.

You can see the evidence from this page 
http://www.pauerhome.com/eclipse/eclipse99/1contact/1contact.htm