My ford Taurus randomly shuts down, then wont start for 2-3 hours. Diagnostic computers can't find anything.?

My Ford Taurus randomly starts shaking, then shuts down, then wont start for 2-3 hours. It then runs normally for a while. Diagnostic computers can't find anything wrong. It happens maybe once every 1 or 2 months. Its a 1999 Taurus with a 3.0 Vulcan (spelling?) V6 engine. It has about 86,000 miles on it. I really like this car and never had a problem with it until now. Any ideas?