Sounds like an end of lamp life scenario.
Arc tube would be noticably darkened, or sometimes a bit warped.

Also may be an indication of:
<OL TYPE=A>

[*]Input AC Voltage Is Too Low,

[*]Incorrect Lamp / Ballast Matchup,
or,

[*]Excessive Heat Trapped Within Luminaire.
</OL>

Don't exclude a "Logical Paradox" setup between the fixture and a Photocell which controlls it! [Linked Image]
(Light from fixture shines on Photocell, Photocell turns off Lamp; No light on Photocell, so it turns on Lamp; Lamp produces light output which is detected by Photocell - Photocell turns off Lamp; No light on Photocell, so it turns on Lamp: Light detected by Photocell..... yadda yadda yadda!!!)
In BASIC, this would look like:

10: P AT 100: "Light On/Light Off"
20: GOTO 10
run
[Linked Image]

Scott35

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!