Emanual,
Which BSC version are you using is it BSCV2.8 or the iBSC? I have some experience on ZTE, can you post your current parameters:
1)Cell reselection PI
2)CRO for 1800 and 900
3)Temporary Offset
4)Penalty time
5)The Rxlev of the serving 900 and the target 1800 after the handover