Why Is My Conversion Rate Dropping?

I ran the same ads to the same landing pages in August and September, yet my conversion rate has dropped substantially this month. Thus My CPA is UP.
-The Average CPC from month to month is about the same, with minor variances only.
-The Average Position Of My Ads In August Went Up To 6.8 From 7.3 For The Most Important Keywords. (This could mean more "explorer" clicks, but the difference s is not that significant)
-I have not made any significant changes to my Keywords or Bidding.

So, in your view, what are the possible factors that could effect a significant montly drop in conversion percentage, given the ads and landing pages are the same?
Your help is much appreciated.
SZ

See original post by CampervanMan