Make sure that the CPU is operating within its specifications. Many boxes are shipped with incorrect settings for CPU clock speed, power supply voltage, etc. Sometimes a if cooling fan is not fitted, it may be ineffective because a cable assembly fouls the fan blades. Like faulty RAM, an overheating processor can cause all kinds of strange and unpredictable things to happen. Some CPUs are known to have bugs that can be serious in certain circumstances. Try not to get one of them.