I had a similar problem until a few days ago when I had the local power company service men out because I was noticing many switch-overs by my UPS. The problem turned out to be a failure of the ground of the transformer serving my house which they replaced.. I also have a ground in my computer room and it apparently was trying to ground the entire power system. Soil conductivity is poor here and line voltages were varying considerably from 80 volts to over 135 volts whenever some appliance in my house started. And I thought it was variations on the power distribution system. Now my line voltage is very constant and the random freezes have gone away.
13 Xfce, Win XP, GA-MA785GM-US2h, Athlon II X2 240, 4G mem, OCZ SSD, WD 230G, nVidia 210
13 Xfce, Win XP, N68C-5, Athlon II X2 255, 4G mem, OCZ SSD, WD 120G, nVidia 7025
13 Xfce, Win XP, N68C-5, Athlon II X2 250, 4G mem, OCZ SSD, WD 1T, nVidia 7025