Help! my computer always black screens!

when I play CF, my computer goes black screen alot when i do some sudden movement, like reload with most guns, stab with knife, toss a grenade, some weapons cause me to black screen when i just draw the weapon and certain weapons make me black screen by just walking, and in EM or SnD when i watch other ppl play my screen goes to close to their body and they suddenly move it black screens. I found a only way to stop this is by sleeping the computer and unsleeping, but this some times cause me to Disconnect or get me killed, i have already lost over 500 death by AFK black screening. With out these black screens my KDR will be over 1.5 but now it is only 1.4. i play on Low deatail and everything set as the worst, like no blood, no bullet trail, no weather affect etc... is there a way i can fix this? please help me and thank you.