you have to isolate different possibilities.. the CSSR is highly dependant on the formula defined by the vendor.. i’m not familiar with the Ericsson one, but there are at least those possibilities :
1. TCH CONGESTION
2. TCH RADIO FAILURE (interference, low coverage)
3. HARDWARE ISSUES (TRX, ANC, TCU)
The CSSR is a very vast indicator, a grouping of different indicators, you have to dig in further to really understand where your issue is, and what action can be taken to reduce it..