Based on what we've done so far, it would seem sensible to use further problem solving techniques, such as Cause and Effect Analysis and Brainstorming to investigate the underlying causes of these two defect types.
Before doing that, however, it might be worth considering the issue from a second dimension. Simply because Sags/Runs occur most frequently does not necessarily mean that this is the most important issue to tackle. Costs, both in time and money should also be considered. For example: