Can't merge two trees in Family Tree Maker

I am ready my throw my copy of Family Tree Maker in the bin! I have now tried to merge my two trees (1600 people into 4000 people) three times and got about half way through the merge process and it shut down. Three times!!! An hour or more each time!!!! It seems the program can only run for an hour or so before it gets tired and had to be restarted to get its oomph back. I wouldn't recommend this program to anyone.
Ancestry.com you need to pick up your act!!