fan troubles perhaps? please help me!!

I have been having with my computer just randomly and oftenly cutting out entirely, i need to cut all power to start it up again. We took it for repair a couple months ago but it just keeps doing it. The invoice said it was extremely dusty and BIOS power management was set to maximum, which was cauing the problems. A couple weeks ater repair i took i apart and cleaned out a lot of dust again, which seemes to help for a while, the computer stayed on for about 45 minutes as opposed to only 15, but it still cut out. Now another 2 weeks later it cuts out about every 20 minutes. I also notices the longer i wait to turn it on again the longer it will stay on. I also checked bios again and power management is disabled as it should be. Im thinking maybe the fans arent running properly which is causing the computer to overheat? I checked the rpm and one of the fans was good and the other was in red letters (warning perhaps?) but i have no idea what rpm it should be at. any suggestions would be greatly appreciated.