Hello! Based on the information you provided, it is possible that the issue lies with the motherboard. However, there could be other factors contributing to the problem as well. Here are a few possibilities to consider:
1. Motherboard issue: Since you have already changed the power supply unit (PSU), and the problem persists, it is likely that there is a problem with the motherboard. This could be caused by a faulty power delivery system on the motherboard, a blown capacitor, or any other component failure.
2. Power button issue: It is also possible that the power button itself is malfunctioning. Check if the power button is stuck or if there are any visible damages. Try disconnecting the power button from the motherboard and using a different method to turn on the PC, such as shorting the power pins with a screwdriver. If the PC turns on without any issues, then the power button might need to be replaced.
3. RAM or CPU issue: Faulty RAM or a problematic CPU can also cause similar symptoms. Try reseating the RAM modules and CPU to ensure proper connections. If you have multiple RAM modules, try using them one at a time to identify if any of them are causing the issue.
4. Other hardware issues: It's worth checking other hardware components as well, such as the graphics card or storage drives. Make sure all connections are secure and there are no visible signs of damage.
If you are not comfortable troubleshooting the issue yourself, I would recommend seeking assistance from a professional technician who can diagnose the problem accurately. They will have the necessary tools and expertise to identify the exact cause of the problem and provide an appropriate solution.