Try to stay away from products containing a huge amount of petroleum distillates. Cheaper products can contain aromatic and naphthenic molecules which are fairly agressive solevents. (uhm, side note: aromatic is used for carb. cleaner and napthenic are used for paint dryer).

Much of it has to do with the construction of the tire. I use Meguiars Endurance (probably the same stuff you have) with very good results. Tires turn brown because they have been cleaned too much and too often. Try to minimize cleaning with stuff like Wesley's (I use it every few months only, between that time I wipe the tires with the wash water after i'm done cleaning the wheels, let the tires dry, and apply the gel). I noticed that my tires never turn brown (Pirelli), neither do my brother's Firestones (and my dad's Bridgestones). My mom's Michelin's and BFGoodrich (go figure, same company) both turn brown. All the tires get the same care and treatment. The brown goes away, but comes back within a few days after. Of course, you have cracking--but it's probably the tires themselves.

 

It sounds like weather rot, but if you take that good a care of them it seems unlikely. Go back the place yo bought them and ask them what the deal is