How would you handle this situation from a Performance Management perspective?

Scenario:

Project manager

She has 3 years on staff
Consistently hard-working, reliable and positive
Strong performer with great attention to detail
Work has been slipping over past few weeks
Missed critical details on three separate projects
Not tracking due dates and following up on late deliverables
Two team members have shared concerns and implied (Michelle) has personal issues

You are her manager. How would you handle this situation from a Performance Management perspective? Be thorough.