zte intracell ho failure related EFR

Hi,
we have below problem, what can be reason?Thanks for support.

-huge intracell handover failure problem at ZTE(%3 decrease) (MSC Ericsson but bsc managed handover failure problem)
-Failure happens at EFR codec
-problem is not NW level , only some cells
-problem not continues, whenever we deactivate EFR solves and after a time re-starts at different cells

rf comments:
1-90% handover type is pbgt for problematic cells (there should not be external interference problem)
2-there is no change for ping pong handover trend
3-all nr handover trend similar(ncc permition , ba list, measurement report process should be ok)
4-icm clean – dl quality samples clean
5-there is no traffic & kpi(drop – cssr) & ul-dl average power & ta graph change (trx power values are also ok)
6-there is no new site & cell down time problem
7-there is no cr-hw change-alarm-door open alarm

Also we deactivate TFO, re-start bsc, re-start hw(which creates codec) but no help.