Thanks for the prompt Frankie. Here's an update. I learned a thing or two about HID ballasts in my research that may be of use to others - probably more than my troubleshooting report in fact.
The upshot is I decided I needed to replace my ballast. My fixture is a 3 yr. old 250W Sunpod, suspended from the ceiling over my 34 gal Solana cube. This holds one DE HQI bulb, and has a remote ballast enclosure with a fan. There are three components to a pulse start magnetic ballast: a magnetic coil transformer (the actual ballast), a charge capacitor, and an ignitor. A friend of mine was able to test the cap and determine that it was good, which is a pity. It is the cheapest component and easiest to source. I suspect the coil itself. It looks a little cooked out. I don't know the typical life of these things. Mine is 3 years old.
Here's where it gets interesting. The magnetic ballasts supplied by Current USA in their HQI fixtures (at least until recently) were ANSI spec M138. These are actually NOT the spec that 250W DE HQI lights were designed for. That would be M80. There is apparently a real difference in supplied amperage, which can affect both light intensity and spectrum. I found this thread which describes the issue:
New Bulb Burn in Question.... - Reef Central Online Community
That thread was very interesting to me - first off because my ballast has apparently burned up after being used for an application outside it's design, but also because a year ago I replaced the no-name 14K bulb that came with the fixture (I bought it used) with a Geisseman Megachrome 14.5K. The intensity was very high. I initially had to raise the fixture higher to keep from bleaching my corals, but I always thought the color was much yellower than it should be, even after break-in. This is exactly the experience of the poster linked to above. I adapted to it and so did my livestock, but I really wanted something bluer, so I recently decided to go back to the Phoenix 14K bulb that I've used in other fixtures in the past. I bought one a week before my problem, and just hadn't installed it yet.
In any case, my reef tank was in the dark, so I needed a replacement ballast quick. Turns out that a direct replacement is no longer available from Current USA. They now recommend an e-ballast, specifically the Lumatek, which has made a big splash on the horticulture scene. This is the specific model that Current USA pointed me to:
Lumatek 250W 120/240V Dimmable / Multi-Wattage Ballast Kit
Coincidentally, my LFS, who I trust, recommended the same ballast. They are actually in the process of switching over their entire store setup to e-ballasts, and they rave about the Lumatek. E-ballasts are very hard to assess, because they typically are not conforming to any specific ANSI spec. Just because they fire an HQI bulb doesn't mean they will give you the performance of the design spec ballast (i.e., M80 in my case). You really have to try one with a specific bulb to see what results you get. There are significant efficiency advantages of an electronic ballast over magnetic, leading to a lot less heat and lower power consumption. They are also totally silent, and may last longer (can find claims both ways on this point). There are many threads online debating the merits of e-ballasts vs. mag ballasts for HQI, and many purists are unwilling to give up the tried and true technology. Marketing claims by e-ballast makers are hard to evaluate (like all marketing claims). Fewer and fewer manufacturers are offering magnetic ballast products though. One of the most popular true M80 ballast assemblies out there, the Sunlight Supply Blue Wave 3 was recently discontinued (though there is still old stock available from retailers). GE Advance, the biggest maker of core and coil ballasts, also recently deleted the M80 spec from their catalog, so it is going to get harder to source raw magnetic ballasts for both equipment makers and DIY'ers. Electronic ballasts are the future of metal halides. That much is clear. Maybe LEDs will make this issue moot eventually.
So, the end to a long story is I bought the Lumatek 250W HQI capable ballast from my LFS (for about a 10% premium over the online price), took it home and wired it up to my Sunpod. It works well. I plan to post a detailed review of it once I have had it up and running and burned in my new bulb. Give me a couple weeks. If anybody has questions about any of this or wonders where I got my information, I'll try to answer for you.
