LAPTOP Help! I BROUGHT FROM EBAY AND NEED TO MAKE SURE ITS A GOOD LAPTOP?

I have a lenovo thinkpad edge. It starts up great and turns on but then after like 15 minutes, i will get one short beep then the screen flickers black, then after a few seconds it turns back on. The light on the side will dim(the red light on top of the I in thinkpad). Do you have any idea what this is?

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