Find a program to keep an eye on your CPU temperature and make sure
it's not overheating while you're using the SDR stuff. If you haven't
cleaned out your CPU fan/heatsink for some time it could be getting
clogged with dust and not working very efficiently.
Shutdowns are usually more about power supplies than CPUs though (these
days an overheating CPU tends to make the PC run slower instead) so you
might need to check out your PC's power supply, perhaps swapping it
with a replacement temporarily to see if the problem goes away.
The CPU temperature monitoring program should also display the power
supply voltages so you could keep an eye on these while you're using
SDR and see whether the voltages start to go out of range just before
everything shuts off.
Either way this isn't really an SDR problem, just a normal PC problem
that comes about because SDR tends to use more processing power than
most other programs.
Cheers,
Adam.