My New 100w Mega Ray is flickering and sometimes goes off then on again a few minutes later is it faulty ?

This is a problem that has been identified with the European 100w self ballasted bulbs, the filament is not allowing enough wattage through and this causes the flickering and the occasional power failure, as the bulb burns in this should subside and stop completely after a week or two, beyond that the bulb could be faulty.  It will not affect the bulb in anyway or its long term stability.  The bulb design is being changed to a 110w bulb which stop this, but there are still a number of 100w bulbs in circulation.

Does my bulb need burning in; someone on a forum I use says they need 12 hour burn in before using them over my reptile.

No, the bulb can be used immediately over your reptile, following recommended basking distances of course.  It is true that some compact lamps need a recommended 12 hour burn in but not the mega-ray they can go straight our your animal.

Do I need a special fitting or holder for my bulb ?

If using a dome reflector you will need a correctly rateed dome with ceramic or porcelain fitting, if using without dome a ceramic or porcelain holder is essential, these lamps produce heat which travels up the bulb and into the holder, by placing the bulbs into any plastic fittings will melt them and could cause fire.  For the Externally Ballasted bulbs the current bulbs have a metal collar, we have found that some fittings stop the bulb making contact inside the holder, but most holders will work fine, its worth checking with us or your stockist before making your purchase.