i just had a 1000w HPS doing the exact same thing as you described... going out... then coming back on, then finally, it doesn't even come back on anymore.
they 'upgraded' me to a AgroStar magnetic ballast... and the guy i talked to said he has fixed over 1000 of these now and its always the same thing, the capacitor is failing and eventaully it takes the ignitor with it. They will either send you the replacement parts for you to do a quick fix. Or you will send it in to them and they will fix it. If its from HTG you're under a 2 year warranty, so that's the good news.
Apparently a fair percent of these ballasts experience this problem. I am told that it shows up rather quickly usually in the ballasts that are going to be effected, and that once fixed, it will run very well for a long time.
good luck, to you and me both.
(before i even called them i tried chaning which power line i was coming off of, changed out the surge protector, the timer, and the bulb. it was deffinitely the ballast fugging up.)