Eh bloke, did you mean Metal Halide or are you actually using Halogen? The best type HID (High Intesity Discharge) lights are MH and HPS (High Pressure Sodium) for there light spectrums. The former is generally used for vegging while the latter is for flowering.

As far as your problem, it would help if you could post pictures to help identify the crux of the matter. Do you have a digital camera?

It could be that godzbudz is right and you are cooking your plants. I use three lights in flowering. My 600w 's are kept at least 15 cm from plant tops while I keep the 1000w light about twice that distance and they are all on light movers. Maybe your light is too hot and too close. You can use air cooled hoods to help aleviate the problem as well.

Could just as easily be nutrient deficiencies. Buy a good grow book to diagnose the major and most common nutrient problems.

Long Live the Queen!