Sensing data seems increased when YUN is running longer period of time

I am working on a project that involves recognizing gestures people perform during interaction with a conductive cloth. These gestures could include no touch, one finger and palm, etc.

However, when I was collecting the sensing readings, for example, no touch, the reading was increased with time. It seems like the reading is increased when Arduino Yun is running longer period of time.

I was collecting the data with 100 samples as per set. The data started with arond 1500 and increased to 9000 after I collected 10 sets of reading. Interestingly, every time when the reading was increased to a much higher value, it always dropped to a remarkable low value and even 0, which is like a signal predicting the sharp arise of reading.

What could be the possible reasons?

And, does it still can be used for Weka data mining?