PPC Conversion Landslide - Exploring a Slippery Slope

Is there ever a time when a paid search marketer can stop optimizing their campaigns and just sit back reaping the benefits of a previous job well done?  This is a question I have asked myself many times over the past seven years. While managing PPC campaigns against various performance metrics including cost per action and conversion (e.g. visitor to action completion), I continually have discovered that if I am not actively adjusting bids, testing new ad copy and optimizing landing page variables (e.g. headlines, opening body text, images) my conversions tend to drop.  I have spent a great deal of time driving up conversion one month to achieve strong rates of 8% - 10% to realize that the following month and without adjusting anything further, conversion would drop 3%-4% for no reason.  I never could figure out why and honestly I haven’t still figured it out.

But I did come across an older experiment conducted by MarketingExperiments.com. The experiment was reported in June 2006 with the title, “Website Conversion Erosion.”  It has helped me start to shed some light on the experience I have encountered and offered a few possible explanations. Definitely worth reading even though it was written two years ago.
Mainly the explanations offered included a decrease in brand awareness from media and an increase in competition. I certainly agree with both reasons for a conversion decline from month to month. Increased competition that knocks your listing out of a strong bid position or generates more attention through a stronger/more relevant ad could definitely hurt visitor quality that affects conversion. Although if you are carefully watching your max bid, average bid position, conversions and quality score, I would assume that only a shot-term and minor impact would occur which could be quickly addressed.
A drop in offline media awareness (either as a credibility or curiosity driver) could hurt conversion as well. I have witnessed the opposite where powerful brand awareness offline drove conversion through the roof so it isn’t unreasonable that a lack of awareness could lower conversions. But I am not sure if either of these explanations adequately fit my past experiences or if they could even be effectively measured (for a small business) to determine a cause/effect.
Seasonality or momentary trends could affect PPC conversions from month to month depending on a particular market. Maybe a change in search intent could alter conversions; although for a national campaign this may be highly irregular without some form of media influence.
I often wonder if there is a momentum factor some how embedded, primarily in Google AdWords where I have experienced the most fluctuation in month to month performance, that exposes ads to more relevant searches. Like some sort of algorithm that picks up higher levels of activity and rewards the hard work. Maybe it’s some magic within the “black box”?
Regardless, the situation is interesting. While researching it, I have never detected any major changes in CPC or average bid position (basically all controllable variables remained constant) from month to month yet conversion still erodes. Have you ever experienced this?  What do you think is a possible explanation?