The MS which are attached to your network does not reselect your competitor for no reason. It means they loose coverage of your network and are not getting it back ! Can you ensure that RXLEV_ACCESS_MIN = -110dBm
MS_TXPWR_MAX_CCH = 33dBm
in the cells nearby the border ?
The MS attached to a VPLMN will try to re-attach to its home plmn every few minutes.