Finder relaunches itself every minute!

For the last couple of weeks the window focus mysteriously shifts away from the finder while I am working with it. Also, the finder window does frequent refreshes - even if it doesn't lose focus. And the desktop icons flicker and refresh quite often. I just deleted the finder plist and the problem continues.. It often closes and reopens, having returned to my home folder. In fact the finder is unuseable, it happens so often. I have a 30 day trial of Pathfinder installed until I can get Finder working again.

Can anyone suggest how to solve this irritation? Thanks a million! John