Makes me think hardware issues. How long has the system been running boinc without issues? Do you regularly clean the dust bunnies out of the heatsinks and fans? If not, take the side panel off and use a can of compressed air, or like I do, a shop vac with the hose plugged into the exhaust port. Do outside though, unless you want to blow the dust back out all over your home and be careful blowing dust out through the fans. If they spin up too fast it may damage the bearings. Or so I am told. I have never had issues with this. Do all fans spin when the system is running? If not, there could be heat issues and the system could be overheating. When blowing the computer out, make sure to pay attention to the intake/exhaust vents. A clean fan will do no good if the vents are clogged. A computer on the floor will accumulate dust much faster than raised up. Especially if there are pets in the home.

What sciences are you running? i.e. is it only a specific science that you see causing the system to hang? One thing that I wonder is maybe a bad section of RAM. If you have all projects running MIP for example min requirements here , the minimum requirements are listed as 650MB of RAM, and if you normally wouldn't access a higher segment of RAM during normal computing and MIP is attempting access, it could freeze the system. Just a random guess though. You could run memtest to check RAM. There are also bootable discs that you could use to check the cpu and other hardware components. Ultimate boot cd is one that comes to mind, but its been quite some time since I have needed to download this and I am not sure if it is still maintained.