Palladino Emanuele

Hi there, 
I am using the Keithley 2400 to collect some voltage and current data using 2-wire connection with 2 probes. I have been running into some issues when collecting voltage vs time as the voltage start fluctuating randomly. 
Current measurements are mostly fine and this seem to happen only with voltage.
This tend to occur when I open the shutter of my solar simulator, but it also happen in the dark.
The fluctuation is not around the reading value, but the reading goes from a value to nothing on the Keithley screen which on my computer appears as a sawteeth pattern.
I have tried moving the probe needle to a different spot but as soon as I open the light it happens again.

​​​​​​Is there any issue with the Keithley?

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