Does anyone use an underground fence?

I just installed one, and am having some problems using it with my dog. I was wondering if anyone might have some suggestions! We have gone through the whole training regimen, and my dog definitely understands where the boundary is and generally respects it.

So far, however, she cannot control her initial impulse to run after something very stimulating (usually another dog or a person she knows and likes). She runs right over the border and barely seems to register the shock she gets. Once the initial impulse has been 'fulfilled' and she is returned to the inside of the 'fenced' area, she won't run out again and won't even go near the border.

They have another collar that delivers a stronger shock, but I just don't feel right doing that. I feel like that shock she gets with this collar should be plenty, as she definitely seems to almost fear it (except when she's running through it!). I would never have done any sort of shock-based collar to begin with, except for the fact that we cannot fence our yard - this is the only option. Walks and runs sufficed before, but I can't seem to get consistent walks in with 2 small kids. As long as she doesn't see another dog walk by, the underground fence has been *fabulous*...she spends hours outside with us in our yard, she gets exercise, etc.

FYI, my dog is a small (55lbs) golden retriever and the underground fence is the Petsafe brand.