After upgrading a CPU, if the system locks up and restarts after 15 minutes, what should you check first?

When a system locks up and restarts shortly after a CPU upgrade, the most likely cause is related to overheating, which can occur if the thermal interface between the CPU and the heat sink is inadequate. Checking the thermal paste is critical because it plays a vital role in thermal conductivity, allowing heat to transfer from the CPU to the heat sink effectively.

If the thermal paste is either improperly applied, insufficient, or old and dried out, it can lead to poor heat dissipation, causing the CPU to overheat and the system to become unstable. This instability can manifest as system lock-ups and unexpected restarts.

While RAM configuration, operating system settings, and video card performance may be factors to evaluate in different scenarios, they are less likely to be the immediate cause of the symptoms described following a CPU upgrade. Therefore, ensuring proper thermal management by verifying the thermal paste is the best first step in troubleshooting the issue.