Random BSODs during normal work

Hi, I'm using my laptop (ThinkPad L512) from 1,5 years and since end of January this year I regularly got BSODs during normal work. There were about 15 of them until now. The only thing that I noticed and think could be useful is that I had about 3,4 of them just after unplugging external monitor. The other ones seems to appear without any specific circumstances (with or without external...