Fig. 1 - Ice Heated to Boil Over Time

Today's lab involved taking data over a 20 minute period.  We wanted to see if the addition of salt made water boil faster.  Half of the class boiled ice and the other boiled it with the addition of a 1/2 teaspoon of salt.  The graphed data is due Monday and should somewhat resemble the line graph above.  

Question:  Why the peaks and valleys before the 5 minute mark??