Initial booting problem, Vista

Intermittent snag. On initial power up, system hangs up before BIOS screen. I reseted RAM Sims and it works, sometimes it has to be done numerous times. At other times, the system will successfully boot but during power up, before BIOS screen, the power shuts off for 2 seconds then comes on again, 2 or three times then it boots normally. I cleaned up everything inside and made sure all connections are in place. I heard that it might be necessary to reset the CPU, not sure about that. Any advice would be welcome.