Run anti-virus AND anti-spyware scans. It could be a network-wide infection caused by one system, or you both may just happen to have malicious software on your computer.
Also, are you running a heater in your home near the systems, or is it hot inside the rooms where they reside? Are any error messages coming up after? Are they just suddenly losing power, or going through the shutdown process? Is it happening randomly, or at specific times?