You will need to keep track of the number of lines that you have printed per page.  I would do this by shared variables and updating the variable everytime a line is printed for the item code and resetting the variable every time a new page is displayed.  Modify the formula for a 'New Page Before' in the Item Code group if this variable exceeds a set value (whatever the 25 lines work out to).  

 

This would keep  the number of lines displayed on a page to a set limit and keep all of an item code together (which is what I would want to do).

 

Hope this helps.

Shared Numbervar GrossAmount; (you can initialize as well, but I only do that in a formula to reset the amounts.)  They are case sensitive.

 

Back to your case:

In the page header the formula would look something like:

Shared numbervar Lines := 0;

""   //I put this in the formula so nothing will display

 

In the detail section:

Shared numbervar Lines := Lines + 1;

""

 

In the group header section under new page before

shared numbervar Lines;

Lines > 20  //the number is not specific, you would want to experiment to see what gives you the best results

 

Hope this helps