There are many reasons this can happen..
overheating
malware/spyware/trojans/viruses
hardware issues
software issues

clcik start ...right click on "my computer....select properties....select "advanced tab"....select the settings tab in start up - recovery area ....remove the check in the auto- restart box.

By doing this the auto restart will quit happening and an error should come up.

write the EXACT error message in this post and we will try to determine what it is!