I2C initialisation problem with MLX90614

Currently I'm developing an Espruino module for MLX90614 - an infra red thermometer I2C sensor.
https://learn.adafruit.com/using-melexis-mlx90614-non-contact-sensors/overview
It's nearly complete but I have strange problems with the initialisation. I figured out that the sensor always works when the power supply will be switched on after I2C1.setup(..)-call. So I have connected Vin of the sensor with B3 to simulate switching on/off programmatically.
The following code works without any problems:

I2C1.setup( { scl: B6, sda: B7 } );
digitalWrite( B3, 1 ); // Power on after I2C1.setup(..)

function start() {
  I2C1.writeTo( { address: 0x5a, stop: false }, 0x06 ); // repeated start I2C requests
  var d = I2C1.readFrom( 0x5a, 3 );

  var temp = ((d[1] << 8) + d[0]) * 0.02 - 273.15; // formula from datasheet
  console.log( "Temperature = " + temp + "°C" );
}

setTimeout( start, 250 ); // Sensor available 1/4 sec after POR

But now strange things happen. The following code works only one or two times after connecting the USB cable and then never again.

digitalWrite( B3, 1 ); // Power on before I2C1.setup(..)
I2C1.setup( { scl: B6, sda: B7 } );

function start() {
  I2C1.writeTo( { address: 0x5a, stop: false }, 0x06 );
  var d = I2C1.readFrom( 0x5a, 3 );

  var temp = ((d[1] << 8) + d[0]) * 0.02 - 273.15; // formula from datasheet
  console.log( "Temperature = " + temp + "°C" );
}

setTimeout( start, 250 ); // Sensor available 1/4 sec after POR

Instead of Temperature = 22.37°C an exception is thrown:

Uncaught InternalError: Timeout on I2C Write BUSY
 at line 14 col 51
i2c.writeTo( { address: 0x5a, stop: false }, 0x06 );
                                                  ^
in function called from system

I have no ideas anymore how to find the cause. I don't have a memory oscilloscope and can't say what happens during "I2C1.setup(..)".
Does anybody have an idea how I could continue...?