While my 4000s have gone on strike several times, it's always been the fault of the connector underneath the 3640 CPU daughterboard. It sits on four nylon standoffs, with no indent and nothing to secure it. If the plug works its way out through vibration, a bump (or a tilt) or just years of alternate heating and cooling, it'll fail for sure.
I'd check that the CPU board is plugged in firmly. At least one of my CPUs has to be held in place with a cable tie to keep it working.
tony