it’s either bad coverage, interference or TRX hardware problem.
you have to investigate more into details :
what is the ratio of quality/level handovers compared to better cell handovers ?
what is the sub-cause of SD drop ?
what is the sub-cause of tch asgn fail ?
can you see the split of drops & fails per trx ?
are those cells having a large coverage, or a resurgence of coverage, meaning that they are covering some subscribers with a very low rxlev ?
do you have access to radio mesurement statistics ?
-pix