lorax - 2:32 pm on Jan 8, 2012 (gmt 0)
First rule of troubleshooting - look for the obvious - but I'd start with more basic stuff than SEO/PPC. Look for issues with the site - test the purchase process and look for 404s, missing images, etc.
Next look at your analytics. Are your keywords still relevant - look for increases in bounce rates on particular keywords over time. High bounce rates indicate either a real issue with the landing page(s) and/or the user isn't finding what they're looking for. This could be because the keyword value has shifted and no longer brings in the type of visitor you originally targeted.
Bare in mind the conditions in the market space and seasonal influences. Some times a world event can cause a uncharacteristic dip or boom.