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Just to as a complement to Majenko's answer (which I second), there are a couple of flaws in your program that may be related to your problem:

• lowest is always zero, as it is initialized to 0 and cannot increase
• your computation of the mean is incorrect: the mean is the sum of the readings divided by their number
• you fail to reset lowest and highest between consecutive runs of 8000 samples
• there is no point in putting all of loop() inside an infinite loop.

Assuming you really intend to print summary statistics on runs of 8000 samples, as your code suggests, I would write it like this:

const int SENSOR_PIN = A0;
const int SAMPLE_COUNT = 8000;
void loop()
{
 long sum = 0;
 int lowest = INT_MAX, highest = 0;
 for (int i = 0; i < SAMPLE_COUNT; i++) {
 int sensorValue = analogRead(SENSOR_PIN);
 sum += sensorValue;
 highest = max(highest, sensorValue);
 lowest = min(lowest, sensorValue);
 }
 float mean = (float) sum / SAMPLE_COUNT;
 Serial.print("mean: "); Serial.print(mean);
 Serial.print(", minimum: "); Serial.print(lowest);
 Serial.print(", maximum: "); Serial.println(highest);
}