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:
[*]Input AC Voltage Is Too Low,
[*]Incorrect Lamp / Ballast Matchup,
[*]Excessive Heat Trapped Within Luminaire.
Don't exclude a "Logical Paradox" setup between the fixture and a Photocell which controlls it!
(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