Well it appears that you have more than one phenomenon going on here; but the most basic thing is the 'warm-up' that is centered on the 10 time lag. If you notice, the phenomenon begins to 'warm' at time 0, indicated by the blue areas getting lighter and then becoming white, and then warming even more into the 5 time lag point. This phenomenon continues to warm to a certain point, and then appears to cool. At the 20 time lag, it appears to be cooling down to the point of the beginning of the test. I would suggest that this might be due to something like exposure to an external, regularly variable, heat source- such as the Sun. Am I in any way, close?