For the purchased parts what are the rules you have set up on the planning screen.

Even though you build 5 days a week you always need to make sure you have enogh of the parts available but if I understand you correctly it is actually the next level down that is causing the issue. 
What is the firm period on the Supplier Schedule because that may be part of the issue.  If you have a supplier schedule with say a 14 day firm period it means that MRP cannot "add" any additional requirements in that 14 day window, so you will probably be seeing a lot of action messages, suggesting you bring orders forward (expidite) but these changes have to be made manually and that means talking to the supplier to see what they can dobecause you are breaking the Firm period.

Another option would be to calculate how often you get these stockouts and what the amount you are short.   then you need to decide if it is financially correct to hold a safety stock of these components to allow for a smoother SS