System down about 7hr in flight PMDG744

Hi to all, I've tried a long haul flight via VATSIM using Jepp WX not from VATSIM. However, for shorter trips such as 3 hrs. I could complete without any system errors. I was on a flight from Seoul to New York. but about 7hrs in flight with a system warning sound the monitor just went black and then I hear the eng running and all different kind of warnings from PMDG e.g. bank angle, stall etc. Or it sometimes just restart the whole computer when I tried to click on the PMDG menu using Alt key during PMDG is loaded in MSFS. In addition, I could complete a long flights with PSS777 without any problems. I use the proper product keys and MSFS not those download files on the Internet website.