What’s Problem as computer reboots after some time automatically?

Hi,
I changed my RAM from 512 MB to 1 GB some days ago.
Every thing is right as application speed is faster and quicker than previous speed but my computer restarts after a while.
Duration of time is about 5 to 10 minutes.
I checked everything like RAM, Processor, Cables etc and cleaned them too but no result. Every thing is working properly but why computer restarts again and again!
Please help me what's problem can be in this situation as I have installed windows many times in PC.
Best Regards,
Junaid Shehzad
