You should verify the quality of your transport network between BSC and BTS. Is it a PDH or SDH, consider that BTS uses abis PCM to extract the clock but if you are using SDH you need to verify the clock reference for SDH network, it should be stratum 1.
Also in PDH, Nokia recommends not to put more than 5 BTS’s into a chain.
Finally, in new year eve we got many 2993 alarms. The problem was related to a severe radio control channels congestion under the coverage of one BSC which caused stop traffic in many BTS. It seems to be a SW bug still under study.