BBQ Rib problem, need suggestions
There are two things that will help here:


"Low and Slow"; drop your heat to 250°F for about 5-6 hours
and
Add a braising liquid, this will generate steam inside your aluminum foil pack. 
Both of these will help to break down the meat to the desired 'falling of the bone' condition.


You can see a demonstration of this by Alton Brown at Foodnetwork.com

Alton provides both a good rub and braising liquid, but I prefer to use beer rather than wine in the liquid and have tried it with other recipes that follow the same 'basic formula'. Mt. Dew or Dr. Pepper are both good choices.