STM32 "Blue Pill" Serial.print freezes Serial Monitor

I am having difficulties when printing any type of variable using the Serial.print(ln) command.

The Serial Monitor becomes inactive after printing for approximately 7 - 15 times.

I currently have the STM32 Blue Pill Board, running Mac OS Mojave and Arduino IDE v 1.8.9

void setup() {
  Serial.begin(9600);
  pinMode(PC13, OUTPUT);

}

void loop() {
  float raw = analogRead(PA0);
  float voltage = (raw * 3.3) / 4096;
  Serial.println(voltage);
  delay(500);


}

The STM32 usb-serial as included in stm32duino is a bit wonky. I have never managed to get a unit send data telia bly for over 65 seconds - but it sometimes starts again after another 65 seconds. For me it looks there is a counter overrun somewhere.