HELP!!! HPS on the fritz any input??!!??

I have got a Sun System 100w enclosed ballast grow light atop my grow box. I let a friend borrow it a few months ago and when i got it back it would kick off every now and then, but then warm back up and keep goin for a while, there seems to be no pattern as to when the light kicks off, sometimes twice an hour sometimes not at all in a 12 cycle. do i need a new bulb maybe, i know the bulb in there is quite haggard and has seen alot of time or do i have a larger problem?? any input would be appreciated. tok'on
 
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