When a system starts but crashes/reboots intermittently

1. The system randomly crashes or reboots for no apparent reason:

• Check for viruses. Some viruses (especially memory-resident viruses) can cause the PC to crash or reboot unexpectedly. If you haven't run your virus checker yet, do so now.
• Check the power supply cables. Verify that the power supply cables are attached properly and securely to the motherboard.
• Check the powersupply voltage. As I have said earlier use a voltmeter to verify that each output from the power supply is correct. If any output is low (especially the +5-volt output), replace the power supply.
• Check the CPU. with all power off, check to see that the CPU is cool, that the heat-sink/fan assembly is fitted on correctly, and that the CPU itself is inserted properly and completely into its socket. If the CPU overheats, it will stall, taking the entire system with it. 
• Check the CPU socket. If the Cpu is seated in a ZIF (sero insertion force) socket, make sure that the socket's tension lever is closed and locked into place.
• Check all SIMMs. With all power off, make sure that all SIMMs are seated properly in their holders and locked into place. you may try removing each SIMM, clean the contacts, and reinstalling the SIMMs.
• Check the expansion boards. Make sure that all expansion boards are seated properly. Any boards that are not secured properly, or that are inserted unevenly, can short bus signals and cause spurious reboots. If you have recently installed now expansion hardware, make sure that there are no hardware conflicts between interrupts. DMA channels, or I/Q addresses. 
• Check the motherboard for shorts. Inspect the motherboard at every metal standoff and make sure that no metal traces are being shorted against a standoff or screw. You may want to free the motherboard and see if the crashes or reboots go away. If so, use nonconductive spacers (such as a small piece of manila folder to insulate the motherboard from each metal standoff. If the system continues to crash or reboot (and all voltages from the power supply are correct). replace the motherboard.