Frist you must check your measurement equipment. May be you phones are broken …
If everything is OK and this happening only few BTS’s , may be there is a problem with BCCH TRX. (Bad RF fiter or someting else).
Do you have declared adjacent’s with ARFCN +/- chanel on this cell ? If you don’t have, but your phone measure ARFCN +/- , problem is clear 🙂