Macbook pro crashes occasionally on dual monitor I'm using a Macbook pro 13' mid 2010 running OS X Lion 10.7.5. I just got a Viewsonic 24' monitor (2450wm-LED), and I connect it to my mbp using a miniport to VGA adapter (this one on Amazon). However when I plug into my mbp, I got some occasional crashes which lead to hard reboots. 

I'm using it as a second monitor, i.e. with my laptop lid open. For from 30min to many hours it works fine, then suddenly I got a grey screen (or white, yellow, blue screen) on both (my laptops and the extra) monitors.

The graphics is NVIDIA GeForce 320M 256 MB graphics.

I tried to do a trouble shoot of my own but doesn't work. According to this my firmwares are updated. Using my friend's adapter leads to similar crash, using her monitor (Viewsonic 2431wm) and my adapter leads to similar crash too. Now I think it must be something wrong with my laptop, not the monitor or adapter.

Has anyone here experienced similar situation? What shall I do? Will using a DVI adapter instead of a VGA one help?
Upgrade your NVIDIA driver according to your OSX version: http://www.insanelymac.com/forum/topic/306535-nvidia-web-driver-updates-for-el-capitan-update-06232016/
After restart you'll see an NVIDIA icon on top right of your desktop
Chose 'OS X Default Graphics Driver'