Not to exclude the possibility that one of the previously substituted units is also bad, there still remain some common elements:

the keyboard or mouse you are using could be continuously sending data interrupts (test them on any of your other ADB systems)

faulty motherboard to hard disk cable (substitute)

faulty hard disk power cable (may need to clean, crimp wire connections and connector contact barrels slightly if it plugs in too easily)

intermittent AC power line cord (substitute)

Also, take a careful look at the inside of the system cabinet, the air intake paths and the sheetmetal surfaces, looking for flaking plating or whiskering. You do not want tiny conductive things being blown around by the air cooling to land between IC connections and cause random shorts. A good cabinet disassembly and cleaning might be in order with compressed air and/or vacuum, with static electricity buildup kept under control, if you have any doubts here. One industry test is to gently apply ordinary transparent tape to metal surfaces (but not to ICs or circuit boards: ESD risk), peel, and carefully inspect the tape with magnification. Any metal particles is bad news.