While there has been NO official acknowledgement of a 'defect' in the earlier 3.5L motors, there have been enough changes from 98/99 to 00/01 to warrant notice.

Stick with a 00 and newer and you'll be free of many of the issues that seem to plague the 99s. Not saying that it'll be "trouble-free," nothing really ever is. You'll just feel better at night.