CSSR values should not be impacted by RACH failures (depending on which formula you use for CSSR)
RACH failure might be due to the use of ABIS over SATELLITE, is it your case ?
otherwise, you should check BCCH/BSIC couples, and also TCH/BSIC couples
finally, it might be due to electromagnetic interference : are there military radars around, or military ships ? what about alarm systems ?
the only way to know : perform an ABIS trace on this BTS.
Regards
pix