UPS units come in several flavors... Double Conversion, Online, and Standby. Standby UPSs comprise 95% of the market... but they really don't do well at backing up magnetic loads, such as motors, ballasts, etc. They will run them okayish... pumps and such will run, but not exactly right because the waveforms of AC they produce is different. Standby UPS units are the ones you get at Best Buy or CompUSA, etc. They are the cheapest units.
To run a halide correctly, you really need double conversion, but be prepared to spend a TON on it. Double Conversion UPSs are designed for server rooms and the like. That said, it's touch and go, and the standby type will usually work either acceptably or well enough that you don't notice any problem, so it's sort of a case by case thing.
Anyhow, back to your topic, your bulb is either failing or the capacitor in the ballast pack is dead. A 4 month old bulb should not be failing, but it might be a faulty bulb that just died early. What I would do is replace the bulb (or swap with another fixture if you have one) and see if the problem goes away... if not, then you have a spare bulb for when the one you have DOES go out....
If it remains, I'd look in the ballast pack at that capacitor. If it has gone bad, you can easily order a replacement from Grainger or any local electrical supply house... metal halide components are very common in the commercial industry so any big supply house (read: NOT Home Depot) will carry them. A capacitor shouldn't be more than 10-15 dollars.
