PAR SENSOR PROBLEMS

Hi guys!
I am trying to use a Par sensor with my arduino uno I have problems with my data.
This one is the sensor

The point is that when I start to run the sketch doesnt have any problem but when I have some big change in the colected data, the lecture doesnt back stable again.

The sketch is really basic

float Count;
float Voltage;

void setup() {
  Serial.begin(9600);
}
void loop() {
  Count = analogRead(A0);
  Voltage = Count / 1024 * 2500.0;// convert from count to data
  Serial.println(Voltage);
  delay(1000);
}

ps: If I upload again the sketch, disconnecting before arduino from a power source, works again until a big change in the reading where start to be unestable

thanks 4 your answers or ideas!

Can you post the schematic (and check the wires?)

all what i have is the sensor connected to GND, an pin as a INPUT and 5V. .. any wire touch another so i really belive that the problems is with the data... could be a overflow?

do you have a multimeter to measure the voltage?

yes, i have. the sensor is not working in low light. I tried another one and work fine! thanks in any case guys.