I have a set of headlights with the same situation. The start of yellowing in a spot, but not a big burn or cracking. With a lot of searches here and other forums, The most common solution is to go with a basic Xenon conversion. I'm not talking about the full conversion to projectors, but simply adding the ballast and bulb.
Pelican Parts DIY
Xenon Kits on eBay
This would also be part of the conversion to the Bi-Xenon projectors if you plan on doing that later.
The other info that I have found from my searches is to be sure that you are using the lowest wattage bulb that provides adequate lighting if you stick with Halogen. (not 100W)