its not the bulb, its a comon problem with htgs magnetic ballasts... i guess, usualy happens within the first 2 months, and i have had it for 2 months... i just have to send the whole thing back and maybe trade up for a digital one.
i got a 4 bulb t5 setup over em now just to hold them over, i kept 2 split sockets with 4 bulbs in there too just to add some light.