Lights Going on and off Switch on

I hope to get some explanation of how to go about determining the cause of this problem, so that I can fix it. It is a light that, when turned on, goes on and off arbitrarily without flickering. The light is positioned over a shower enclosure and wired into the branch circuit. Other fixtures on circuit are functioning ordinarily. The fixture is enclosed in a manner to protect the lamp and socket from exposure to moisture from the shower. For some reason the light will go on and off arbitrarily over time. Owner feared it was due to water exposure as vapor from the shower, but the light was observed to turn off about a minute after it was turned on even when the shower was not and had not been running. The light would also come back on in spite of the fact that the water was running, so that does not appear to be related to the problem. It goes on for a matter of several minutes and then goes off for the same, in spite of the fact that the switch remains in the on position. When it is lit it appears to be functioning normally, without any flickering. When the switch is off, the lamp remains off.