Check
1) Power supply voltages
2) CPU Heating up or not
3) Conducting paste on Processor

Clean
1) Processor fan
2) Processor Heat sink
3) RAM and its slot
4) Any card (Graphic,LAN or Modem etc) installed on PCI or PCI-e slots.

Scan and Repair
1) chkdsk on HDD
2) for viruses on HDD

Reseat
1) Processor
2) RAM
3) Any card (Graphic,LAN or Modem etc) installed on PCI or PCI-e slots.

Shuffle
1) RAM in slots
2) VGA (external)

Change
1) RAM
2) PCI Card
3) Processor

Agar aap koi external cards use ker rehe hein, tu woh bari bari utar ke check kerein.