Burning Out HPS Lamps

Into week 2 of bloom phase on an indoor grow I just had my second HPS lamp burn out on me. First one was a Grow Bright from HTG.com. This would be the second grow with this lamp.
At first lamp not coming on a few mornings in a row. Cycling on/off a few times got it to start. After 4 days it was toast. Put my MH lamp in and ordered new Sun Master HPS lamp.
After 5 days new lamp started doing the same thing (not starting up in the AM) and now doesn't light at all. Back in goes the MH lamp and that works fine. Called HTG and they said their digital greenhouse 600 ballast must be fine cause it it works with MH lamp it would work with HPS too.
They were nice and agreed to warranty the HPS lamp and even paid the shipping for the replacement. These guys are very reputable. Here is the question.
Has anyone information or experience that indicates it is a ballast problem killing these HPS lamps or is it a "shit happens" thing? Gonna have to keep a spare HPS around for the next 10 weeks?
Any ideas from electronic wizards reading this thread?
Thanks
Jaybo

I think it maybe the ballast. I am not familiar with that brand of ballast but I have used a few different kinds and I only had one that lasted.

http://wegrowhydro.com/store/product.php?productid=902540&cat=266&page=1

All the others eventually had problems like you are having. Also you may want to go to a quality bulb like a Hortilux or ushio as they have been tested on the digital ballasts and that is what the digital ballast companies generally test their ballasts on.

http://wegrowhydro.com/store/product.php?productid=901570&cat=270&page=1

I agree a lamp that has been tested by ballast makers should be used. But here is an update on my hps lamp problem.
After running my MH for a few hours I put the HPS back in to see what would happen. Guess what?
The lamp lite after the usual 30 second delayed start up. So I called HTG.com and this time I might have found a guy who knows something about this problem. He tells me poor grounding sometimes results with COOL TUBE reflectors and the ground gets loose. He also suggested they have had this issue before.
I took another reflector socket and rewired the COOL TUBE. All connections are tightened with ring connectors instead of slip on connectors. I cancelled the order for the replacement lamp. Tomorrow at seven am I will know if this was the fix.
Jay

GREAT NEWS...Replacing the ceramic socket with one that didn't use slip on connectors resulted in instant start up this morning. After nearly a week of HPS ignition failures start up via timer first thing this morning is showing the ground or poor current carrying capacity of the cool tube factory socket appears to to the problem.
Clearly quality standards for some hydro gear is unpredictable. Knowledge by resellers is highly variable. Next grow I will be looking for a new air cooled reflector. Cool tube works but it is junk. My view is hydro resellers have no issue selling junk if people don't know any better. And it looks like you need a back up for everything to be sure you don't get screwed. At this point I think my HPS issues have ended.
Jay

it sounds like the ignitor is toast thats why the mh works but not the hps.... hps have to use an ignitor to fire up the lamp thats why you can use mh in hps ballast but not the other way around.... have you ever seen the switchable ballast that have the hps/mh switch??? all that does is turn the ignitor on or off.... hit htg with this bit of info and get them to send you a new ballast

where can i get one of those sockets