I agree that without maintenance an AdWords account will exhibit this kind of decline, but it appears to me as though the OP might not be bidding on exact match

I agree that without maintenance an AdWords account will exhibit this kind of decline, but it appears to me as though the OP might not be bidding on exact match search terms which can make it difficult to control your spend and target your audience. If most of your spending is on broad or phrase match keywords then you're far more susceptible to changes due to other people coming into the market place and also have much less ability to optimise your ad text and landing pages to get the best ROI. In my experience the single biggest improvement you'll ever see to your campaign comes from bidding only on exact match keywords that you've extracted from the search term report with ad text written specifically for those keywords. It's a pretty laborious task but the results can be pretty phenomenal.