after 3 years of flawless service one of our bigip 3600s is rebooting at random

i tried - unfortunately the system reboots while running them too. and not at any specific point. it's totally random regardless of what's running or not running. local hardware vendor who resells used bigips says if it's not the memory (which i'm pretty sure it is not) then it is something proprietary and not socketed, like the CPU which is built into the mobo.