They especially do it when turned on first thing monday after they have been in the damp factory over the weekend......after a few hours of being left on....you turn them off, then on again, and they are "cured".
Sound like a loose or dirty internal connector or buss connector problem.
I would remove cabinet, pull out cards, clean edge connectors with alcohol or
contact cleaner, and reseat for starters.
Any dip packages in sockets, carefully pry up a little each end and reseat by pushing
down on them back into socket. You don't have to pull all the way out, like just 1/8" movement.
That helps wipe the contacts again.