Well, the first obvious is, the power supply that you have checked.
The other would be to determine if the crystal is oscillating at its rated frequency.
Look for obviously burnt or discolored components.
Inspect solder joints. Many times cold-solder joints make intermittencies.
Buy yourself a can of freeze spray. Spray at different components at the time to weed out intermittents.
Measure each resistor, one by one, with your DMM and check the resistance value.
Likewise the diodes. And bipolar transistors.
Pray.
But without a schematic................................... I can't really offer a good analytical advice.