One possibility is that it gets overheated specially if CPU working overtime with intensive scan engines. However, it only seems to happen if I try to scan th system. My suspecious mind suggests that there is a virus that shuts down the scan as soon as the program looks for it. It then changes itself so that on a second try the scan does not fid it. It has happened with the MS Malicious Software Removal Tool, with SpyBot, and with the aspersky 9.0 Vius Removal Tool. Nothing was found, including in a scan with MalwareBytes. The system seems to run fine, except for the sudden shut downs. EventViewer throws up an Acpiec error (1)which could be connected to power.
However, if you have a chance of looking at the OTL logs for anything strange, I would be very grateful.
