Your computer problem is strange but very common. You have changed the SMPS and RAM as well as all the cables inside the cabinet. So the only problem areas remaining are the motherboard and hard drive. Please try cleaning all the connectors, slots and components (carefully) using a soft brush. Remove the CPU fan and wipe off the old CPU thermal grease. Then apply some new thermal grease and remount the fan. Also clean the CPU fan properly before remounting it. Check if the problem goes away. Check if all the cables to and from the motherboard and all the internal peripherals including the hard drive and optical drives are firmly fitted and not loose in any way, especially the SATA cable to the hard drive. A SATA cable that is not properly connected could indirectly be triggering the problem. Change the cable if needed.

Finally if everything is fine and the problem still occurs, try using the entire rig without the cabinet. Carefully unscrew the motherboard, power supply and hard drive and assemble them on a table. Place some antistatic foam between the motherboard and the table. If the problem is solved, your cabinet is the issue. Check for loose screws and uneven surfaces inside the cabinet which may cause shorting problems. Clean the cabinet thoroughly too. If the problem still persists, then try reinstalling the operating system. Finding a fault in such scenarios can be quite painful and time-consuming. Be patient and you will get to the root of the problem.