black screen while surfing

while surfing safari i have noticed a strange problem.
My macbook screen goes black for half a sec and comes back.
It doesnt happen usually, maybe 2-3 times a week.
It also doesnt goes to sleep,it just blink for a sec.

i think it started when i decided surfing with safari and not firefox..

if its related to software i dont mind, but im afraid its hardware problem.

apple hardware test(full) says everything is fine, and also geekbench and xbench.

thx! sorry for the bad english...

macbook unibody 2.4ghz\4gb\9400m

Try resetting the System Management Controller (SMC). See the following Apple KB article: Link If that doesn't work, give Apple support a call as it could be logic board related.

Regards.

apple did their "apple test" (something that took 6 hours)on my macbook 2 months ago and everting was fine, so i dont think its the logic board.

is there any chance its related to the 10.5.7 update?
cause i have never seen those black screens before the update, and my macbook is only 3 months old....

anyway i installed the combo 10.5.7 again(last time i updated from the "software update")