If the instability occurs after a few hours in electronic devices it usually is somehow related to temperature.
Electrolytic capacitors are prone to aging so they might need replacing. Especially if they have been used heavily.
If the electrolytic caps have lost much of their electrolyte the charge that they can carry is diminished. As they heat up this would become worse.
Read more here...And even more here...A more remote possibility is a failure of the voltage regulators for the supply for the 68030. If voltage regulators experiences in its lifetime over voltage (more than +35V) it could fail. I don't think this is the case here, as I believe the caps are problem here.
Best of luck and do tell what you find out...
