The icecap 660 is also designed more for T12 VHO bulbs and do overdrive T5 bulbs that could be playing into it somehow but I doubt it.
if you want one of the best T5 ballasts out there as a replacement, the Phillips Advance ICN-2S54 is excellent and will not overdrive the bulb but give equal output.
They are about $40 on Amazon with free shipping.