hi Jembut,
I know it might not help, but it might give hints :
can you compare the radio QoS of those 2 BTS ? especially the call drop rate due to transmissions problems (or TC problems, whichever you call it in your system) and the TCH assignment failure rate due to BSS/hardware/TC.
It will tell you which BTS is affected in reality by the faulty link. If only BTS 2, then link 2 is faulty. If BTS1 and BTS2 than link 1 (and possibly link 2 also) is faulty.