9500 lights

First off I would like to say hey to everyone and share the thought that I have gained lots of knowledge from this site over the last few years. I now have a problem of my own that I cant seem to figure out. I have a 89 9500 that's having issues with the lights. The flashers work fine, however any setting on switch that activates the headlights and taillights ultimately result in my lights flickering. For instance I was picking corn last night and the lights worked fine four about 45 mins to hr. Then started flashing and would stay off more than on. Sometimes it only takes a few minutes for the problem to occur and others it may take 30 40 minutes and I have run a few nights that I didn't have a problem at all. Any advice would be greatly appreciated guys. The board up by the tank has been replaced.