Hi Nam,
I would suspect coverage issue (low RxLev) and possibly a TRX issue. Check on which TRX is set the SDCCH timeslot. Exchange the SDCCH slot onto another TRX..
if it’s still bad, then it’s most probably a coverage issue (since you ruled out interference).
If it’s better on the other TRX, then it’s probably due to a faulty TRX.
Good luck,
Pix