50% drop in Earnings per Click since November

[garyfugere]

In November my earnings per click were at 8 cents per click, but so
far in January I have averaged 4 cents.

Traffic levels, Impressions and CTR are all comparable. And nothing
major has changed on the site.

At first I thought it might have been a post Christmas bidding dip,
but now I'm beginning to believe it is not, as I just looked at my stats
from last year and I did not experience a similar decline.

What could cause such a decline and is there anything I should be
doing about it?

[maxgoldie]

There could well have been some changes that G made, as there are numerous threads on WebmasterWorld and other similar forums where people complain of the same thing since mid Nov.