head light issue

[31sonic]

A problem has shown up where with the headlite switch in "auto" where they turn on when it becomes dark . i started truck one nite and drove to store , came out of store started up truck headlites came on but while driving a few minutes suddenly they went off!!! turned headlite swith to manual setting and back to auto and they came back on ? this has happened a few times lately! what could it be ? the sensor on dash going bad , headlite switch going bad etc. any one have any ideas?