Could someone tell me what might be the cause of a mercury vapor light to
intermittantly go off and in a few minutes come back on?

Curt-

To add to Sam Goldwasser's comments, it is possible that a very short glitch in
the AC power, is causing the light to go out. Once it is out, it has to warm
up before it comes back on. I think a good lamp would have the ability to
withstand a short outage without having to heat-up again.

Perhaps this is due to a defective lamp or one that is getting old. Has it
always done this, or just started after several years of service?

If it has always done it, could you have the wrong wattage lamp for the
fixture? I think the ballast has to be matched to the bulb, just as with
flourescent lamps.

Fred