Reseat all the ribbon cables and press on the asic chip on the cpu board to ensure it's seated firmly (it will make a crunching sound but that's ok). How do the GI connectors look? Are they beginning to look burnt or have you ever changed them? Could also check the voltages at the test points at startup and when the issue occurs to look for voltage drop. My guess is it's more of an issue with a connection than a failing part given the symptoms though.