Got a cupla 70 W jobbies to light up what the city won't light up, and after about a year, one goes off, on, off, on with that whitish glow, rather than the sodium yellow. Is this the bulb going bad, or the ballast? What pita, given how high they are mounted.
The sodium is perty neat, tho, as they are about 7x more efficient than incandescant, and the yellowish lite is soft-ish. The bulb also supposedly lasts a really long time. The bright white parking-lot type lites are metal halogen, about the same efficiency, iirc, but the fixtures are about double the price of the sodium, as are, probably, the bulbs themselves.
Typical end of life behaviour for a high pressure sodium lamp. "Cycling". Ballast should be still ok. Don't leave it in this condition long though (swap out the lamp for a new one asap) as the ballast can overheat.
For those not familiar with high pressure sodium lamps, "cycling" is the when the HPS lamp repeatedlys starts, warms up and then goes off within a few seconds/minutes. If the lamp is not replaced promptly, the cycle gets shorter and shorter. As JB says, it's an indicator that the lamp is at end-of-life. Suspect a ballast failure if the lamp doesn't flicker or start its warm-up cycle at all. In that case, the starting circuit is the most likely culprit and it can often be replaced separately from the rest of the ballast.
Another indication of a bad HPS lamp is a dark yellow or brown look to the bulb. That happens when the arc tube leaks and sodium coats the inside of the outer bulb.
Change the lamp when you notice it first start cycling - every half hour or so. As the bulb ages it requires more power to jump the gap in the inner capsule, and as it warms up it wants more current to stay lit than the ballast is willing to deliver. Poof! Out go the lights. Cool off, try again.
No, the ballast doesn't overheat, you just run out of the fixed lifetime supply of "Magic Sparks" in the Ignitor when you let it cycle like that for months and years.
If you can't fix it now, DO NOT just remove the lamp - kill the power to the fixture till you can. It's still using up the ignitor.
Some fixtures have a separate ignitor in the ballast compartment - most of them are 3 leads, a very few only two and it's an open circuit board. Those you can replace separately ($20-ish for a 'universal') - Others it's all built into the ballast windings and you have to replace the whole thing ($80-ish and up.)
And it's an "Advanced" DIY job, because it gets really hot in there and you have to use special materials and techniques or it'll get cooked and short out. And those sparks are 4KV, put your fingers in the wrong place and they will really bite you.
If the outer envelope is clear, you can also get an idea of the age of the bulb from the permanent dark ends of the inner arc tube capsule. You shouldn't change an old lamp on that alone, they can be black and still work BUT it's getting old, "When in doubt, try a fresh one."
But if it's nice and clean on the ends of the inner capsule, you can trust the guy before you when he said he already put a new lamp in. Now whether it's a quality lamp that actually works remains to be seen
- I hit No-Name crap lamps Bad Out Of Box all the time.