Here are some possibilities:
1. Check your frequency plan and make sure it’s good…this is fundamental.
2. Check your hopping parameters (HSN and MAIOs, hopping reuse pattern, etc)
3. Make sure you use regular antenna azimuths throughout the network as much as possible. Also make sure your cells are not overshooting.
4. Activate power control and DTX if you haven’t done that already.
5. Check for faulty TRXs.
6. Check your network synchronisation
7. Check for external interference.