ps3 shuts off itself

hello there, my question is my ps3 fat 60gb model turns off itself some times while playing selected games, i cleaned it inside out there was too much dust, even its clean but its still doing the same, do i really have to apply new thermal past and heat patches ? and do i really have to use the heat gun? even though there is no YLOD ? other applications working just fine!

an answer would be appreciated.