Arduino to Raspberry Pi serial communication creates only random chars after a few seconds

For my project I need a Raspberry Pi to communicate with several peripheral components, two of them are Arduinos. The one which causes my problem is a Pro Mini 3.3 V ATmega328. The Arduino receives input from two sensors and transfer the data to the Raspberry Pi via serial. A Python code with the serial-package is used on the Raspberry which establishes a connection every 50 ms.

When the input to the Raspberry is printed, the first few lines are correct but after about two, three seconds the printed lines are random chars.

My Python code looks like this:

import serial
ser = serial.Serial('/dev/ttyS0', 115200, timeout=1)
if ser.isOpen():
    ser.close()

ser.open()

...

# loop
try:
        ser.write("1")            
        ser_line = ser.readline()
        print ser_line
...

The Arduino code:

#include <Wire.h>
#include "SparkFunHTU21D.h"
#include <FDC1004.h>

FDC1004 fdc(FDC1004_400HZ);
HTU21D myHumidity;

int capdac = 0;

void setup() {
    Wire.begin();
    Serial.begin(115200);
    myHumidity.begin();
}


void loop() {
    String dataString = "";

    dataString += String(myHumidity.readHumidity());
    dataString += "  ";
    dataString += String(myHumidity.readTemperature());
    dataString += "  ";

    for (uint8_t i = 0; i < 4; i++){        
        uint8_t measurement = 0;
        uint8_t channel = i;

        fdc.configureMeasurementSingle(measurement, channel, capdac);
        fdc.triggerSingleMeasurement(measurement, FDC1004_400HZ);
        //wait for completion
        delay(15);

        uint16_t value[2];
        if (! fdc.readMeasurement(measurement, value)) {
            int16_t msb = (int16_t) value[0];
            int32_t capacitance = ((int32_t) 457) * ((int32_t) msb); 

            capacitance /= 1000; //in femtofarads
            capacitance += ((int32_t) 3028) * ((int32_t) capdac);

            dataString += String(capacitance);
            dataString += "  ";

            int16_t upper_bound = 0x4000;
            int16_t lower_bound = -1 * upper_bound;

            if (msb > upper_bound) {
                if (capdac < FDC1004_CAPDAC_MAX) 
                    capdac++;

            } else if (msb < lower_bound && capdac > 0) {
                capdac--;            
            }
        }
    }

    Serial.println(dataString);    
    delay(20);        // delay in between reads for stability
}

The output of this loop looks like:

So the output loses accuracy and become random chars after about six lines and the output doesn't recover. When I print the serial output in the Arduino's serial monitor the output stays correct all the time. After several tests I run out of ideas. Has anyone a solution for this problem or experienced a similar behavior?