I would, just to rule it out, make sure the ground from the flasher box to the firewall is a GOOD ground...clean any corrosion where its mounted...use a toothed lockwasher to give it bite into the metal.
When its acting up, would be worth checking to see if the 3 lever light switch is still giving power to the flasher unit. Might be able to duplicate the issue by wiggling wires around while reading an electric meter...it could be in the wires or the 3 lever switch...
Hopefully its easy to find...
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!