A PSU (power supply unit) is a component that converts the AC power from the wall outlet into DC power that the PC components can use. A faulty or failing PSU can cause random system crashes, as well as other problems such as noise, overheating, or no power at all. A PSU can fail due to various reasons, such as age, dust, power surges, or poor quality.

To troubleshoot a PSU, the technician can use a multimeter to measure the voltage output of the PSU connectors. The voltage should be within the acceptable range of the specifications. For example, the 12V rail should be between 11.4V and 12.6V. If the voltage is too low or too high, the PSU is likely defective and needs to be replaced.

Alternatively, the technician can try swapping the PSU with a known good one and see if the system crashes persist. If the system works fine with a different PSU, then the original PSU is the problem and should be replaced.

The other options are not likely to fix the random system crashes. Testing the fan speed may help to diagnose overheating issues, but it is not related to the PSU. Disabling the USB ports may help to eliminate potential conflicts with USB devices, but it is not related to the PSU. Decreasing the RAM timings may help to improve performance and stability, but it is not related to the PSU.