Just downloaded the trial and already a problem

I installed Kutools tonight to convert the case in a few spreadsheets. It worked fine. Then I decided to try to combine the sheets into one. The are very large files (21,000 records, 3,100 records, and 2,400 records.) I was able to get through step 2 with one file. It uploaded. Then I tried to add another file, and Excel hung for several minutes and then crapped out, and Excel prompted me to close or restart. I closed and started over. Then I could not even load the first spreadsheet into the combine utility because Excel froze again. Not a great start. Thoughts? I am on a deadline and I need to combine the sheets! Help!