Getting 100% accurate readings all the time when using $10 worth of hardware is mostly wishful thinking I guess 
I too had random spikes in my otherwise so pretty graph from time to time that I couldn’t derive to something specific. So instead of doing a ton of trouble shooting I just added some “sanity code”.
Readings that deviate more than +/- 30 degrees from the previous are simply ignored and a notice is printed in my terminal window:
void startSensorConversation() {
DS18B20.requestTemperatures();
newTemp = true;
}
void getSensorData() {
if (newTemp) {
float temp1E = DS18B20.getTempC(sensor1E);
float roundedValue1E = ceilf(temp1E * 100) / 100;
if (roundedValue1E < (lastValue1E - 30) || roundedValue1E > (lastValue1E + 30) ) {
terminal.println("Error reading sensor 1E !");
}
else {
lastValue1E = roundedValue1E;
Serial.print("Bedroom: ");
Serial.println(roundedValue1E);
Blynk.virtualWrite(6, roundedValue1E);
}
float temp4B = DS18B20.getTempC(sensor4B);
float roundedValue4B = ceilf(temp4B * 100) / 100;
if (roundedValue4B < (lastValue4B - 30) || roundedValue4B > (lastValue4B + 30) ) {
terminal.println("Error reading sensor 4B !");
}
else {
lastValue4B = roundedValue4B;
Serial.print("Living room: ");
Serial.println(roundedValue4B);
Blynk.virtualWrite(5, roundedValue4B);
}
newTemp = false;
}
}