Excel program keeps freezing
 

I'm having a little problem with an Excel workbook that I created for
conference registrations. The workbook consists of 2 sheets. The 1st sheet
has Session Codes, Names, Dates, Times, Costs and Enrollment. The 2nd sheet
has Registrant's information which I group, filter and copy & paste to a Word
doc so each attendee has a Class Schedule. I rechecked functions, syntax
etc. and everything seems okay. I group the info and filter the sheet so
only one attendee's name, date, day, times and class name are shown. That
works okay and I copy and paste it to a Word doc template I created for Class
Schedules. The problem occurs when I go back to clear the print area, ungroup
and show all attendees. For some reason the program keeps freezing. It's
bizarre. Whenever this happens I have to use control-alt-delete. Sometimes
I have to reboot my computer. Anyone know what might cause this?