I agree with Mohttat, I think it is a radio problem : your rf load is too high. It’s clear that this will cause a problem.
Why at bsc border ? It’s probably a coincidence.
Of course, you can support your assumption (that it is NOT a radio problem) by sharing some qos data : CDR for each cell, TCH traffic per cell, and mention which cells are located at bsc border, among the list.