Reboot my Macbook Pro when using external monitor w Thunderbolt to VGA adapter

I have a Macbook Pro running 10.9.2 OS X (MacBookPro8,1). I never had any problem w that but yesterday i bought an external monitor (kind of old model from LG - L1900E). Well, i was trying to extend my Macbook Pro display through a Thunderbolt to VGA adapter. It worked at first but it started rebooting my Mac. I just disconnected that and the reboot stopped.

It sounds to me like a Software related BUG regarding connection or something strange.....

I would like to have some help on how to proceed w that ? Is there a specific SW i need to install to use that ?

I am kind of disapointed because i heard that one of the major items the new OS X was about external displays.....

Looking forward a help from an apple specialist, Tks !

Marcello H. Ishida.