The zooxanthellae will leave the host coral (causing lightening of the coral) if proper conditions aren't met. Zooxanthellae are photosynthetic, so they may be leaving the host coral for a number of reasons:
1. Too much light (which I highly doubt, as I'll explain later)
2. Running too much GFO
3. Too few nutrients (corals are starving)
4. Too little flow, so the nutrients aren't reaching the corals (keep in mind that most corals are about 95% photosynthetic.)
5. Stress from fluctuating temperatures (or too high/low temp)
6. Changing carbon source
7. Changing to an inferior salt mix (not enough trace elements)
8. Fluctuating salinity
9. Changing anything too often, too fast
10. Your tank is too new (my best bet)
First, PC's have earned a bad rep. It's not all about the lighting. I successfully kept SPS in a 40g under 4x65W lights (wouldn't recommend it, but I tried it...and it worked.) Second, "watts per gallon" is one of the worst things I've heard in this hobby. That tells you absolutely nothing about the lighting or the tank. How high are the lights? How deep is the tank? How high are the corals in relation to the lights? A better ratio is the wattage to the surface area of the top of the tank, but that's about the best we can get without a PAR meter.
If some of your corals are browning out, they could be getting too many nutrients and/or not enough light.
My best bet is that in all honestly, your tank is just not quite mature enough for corals yet. It takes a few months (even if you get lots of live rock, live sand, and water from a friend) for everything to even out and be a good system. I hate saying this, because this is the one that no one wants to hear, but it might be time to slow things back a bit. Throwing an SPS into a month old tank is just not ideal. I really don't recommend trying SPS in a tank less than a year old. Things just aren't stable enough.
There are just too many things that could be the source of the problem. Unless you've recently changed something that stands out to you, it may just be trial and error to find that one key trick. In the process, you'll probably change just about everything...and magically, it'll work again. You may never know what it was.