boundary element

Dear BEM experts:

I am trying to find a boundary element solution for the problem of a cavity in an infinte space. The boundary condition is such that the cavity boundary is subjected to a constant radial influx. I have used the standard fundamental solution for the interior case (finite domain). The results, I obtained for the potential were reasonable for points inside the exterior region. However, on the boundary, the results for the potential were in the order of 10^12 (instead of zero value)!!

Any idea what went wrong. Thanks.

Husain