I have the same problem with my core stabalizers- Man are they hard to strengthen. I had trouble with pain keeping me from getting any real improvement untill a week ago. The breakthrough was kind'a cool and really simple once I thought about it.

Do your stability exersized in a lake, submerged up to mid-chest. Some friends and I were goofing around at a local resivior, trying to see who could hold horse stance the longest while working through the basic blocks and punches. This sound very easy untill you realize that the water was really rough from water craft wakes. And the sand shifts around under your feet. Usually my hip flexors keep me from being able to do this for more than 2-3 minutes. In the water I could do them for over an hour.

Man did my legs and trunk ache the next day! I hadn't had that good of a total body workout since before I dislocated my hip. And I noticed the benifits the very next time I tried hand stand push-ups. I have never been good at them, have to use the wall to keep up long enough to do even one. Two days after the lake incident, I was able to do five without touching the wall at all. I then walked on my hands away from the wall about 15 feet.