A PC should be able to run 100% CPU for an unlimited amount of time, no matter what the graphic, energy saving etc. settings are. Actually, those settings have nothing to do with the CPU running at full throttle.
If you're experiencing shutdowns, you're having a thermal problem. If all fans work ok, most likely the cooler itself isn't properly fitted, or the whole interior is getting too hot (had that with a DELL minitower, which crashed after 1.5 hour of video decoding .. with the HDD reaching 60°C case temp!!)
Use HWMonitor to check the temp readings on CPU, GPU and HDDs, and pls. report them here.
If you're very confident that everything is allright, test your right with
Passmarks BurnInTest
While it is buyware, it has a 30-day trial period which should be sufficient for your problem. Download whatever version you like, install it.
Start the software, deselect all but the CPU test, crank that one up to max., fry your PC!
Means: I don't know if the newest version of BurnInTest has an intergrated temp display, so keep HWmonitor running and shut down BurnInTest if the CPU temp approaches 80°C ...