Same banner, same lp, same traffic source. And i notice that ctr goes in the range from 13% to 23%. So my earnings fluctuate as the result as well.
Im starting to bealive in strange theories like that someone else has access to my hosting or prosper accounts, and changes my links with his links from time to time))
Not that odd at all with adult traffic.
Do you find it happening between a certain timeframe? Could be someone outbidding you during those hours and getting the lion-share of the traffic for that spot.
Is it RON traffic? From my experience this is a common result of how some traffic sources algorithms will display your ad and in consequence produce this.
Did you rip the lander? Check if there's no obfuscated javascript playing tricks on you.
Someone else running the same lander / banner combo? This can produce those effects if the traffic source conditions are right.
The most important question is missing : what volume is this? How many clicks per hour? The lower the volume, the bigger fluctuation from hour to hour can be considered normal.