You don't state whether these machines are laptops or desktops, although I suspect it may be an overheating issue. What I would suggest you do, is to cleanout the air vents with compressed air (there is a 'clean out those dust bunnies' thread somewhere in the forum), as well as installing some sort of temperature/throttling software (TThrottle is one I, and many others use - again, details of it are in the FAQ's). With this, you can set BOINC to run full tilt and let the TThrottle software control the throttling of how fast the BOINC software runs depending upon the temperature ceiling you set it to run at. This will also smooth out the 'peaks and troughs' in the rather crude manner BOINC attempts to (say), run at 60% (3 seconds on, 2 seconds off).

It could also be the power supply(psu) if you are still using the stock one, stock psu's are just barely enough to do the job in order to save every penny and then when the pc is pushed to its max, as crunching does, it can be on the very knife edge and anything can push it over causing the machine to shut down. You didn't say if you are crunching with the gpu but that will also take alot of power that the manufacturer didn't think would be happening when the pc was made.