Why Do Programs Keep Crashing

i dont know why but programs are always crashing on my desktop. it's really frustrating. games will also freeze or crash on me abruptly as well. is there a way to know what's failing? graphics or ram or something else? i've built and everything is working great but i think when i switched from 1 monitor to a 3-monitor setup i've been having more problems. i'm not 100% sure but that's all i can think of. a hardware diagnostics test of some sort would help give me an objective answer though.my desktop is a z68 mobo, i5-2500k stock, 8gb ram, and a gtx460 (768mb). i'm connected to 3 displays. the first is 1920x1080 and connected to the gtx460's microhdmi. the 2nd is 1920x1200 connected to the gtx460's dvi. the 3rd is connected to the onboard hd3000 dvi port. there's no visual lag or stutter when moving windows or anything. i can play 3 concurrent 1080p Internet videos on all 3 monitors without any lag. but when i'm gaming on high settings such as borderlands 2, it will just crash on me. it's not consistent enough to know what's causing it but my habits are to game on the 2nd monitor with windows media center playing live cable tv on the 3rd monitor and browsing and chatting on the 1st monitor.

is it crashing because i'm doing too many things that my graphics card can't handle? if so, why is it crashing instead of stuttering?
i also have a problem where if i'm gaming and then press alt+tab to minimize or switch windows, the graphics card will get messed up or downclock or something and the game's fps will drop drastically to like 5-10 fps until i reboot the computer.i also get the bsod sometimes where it says something about physical memory dump.