the causes of internal failures are :
1) Whenever an 48.058 ERROR REPORT message with a cause value of ‘O&M intervention’ is received on Abis interface after TCH seizure and leads to a loss of call.
2) Whenever an 48.058 ERROR REPORT message on SAPI 0 with a cause value of ‘message sequence error’ is received on Abis interface after TCH seizure and leads to a loss of call.
3) Whenever a LAPD failure is reported to the Layer 3 of the BSC (for an RSL supporting a TCH transaction after successful TCH seizure) and leads to a loss of call.
4) Whenever a TCH call release is initiated by the BSS O&M FAULT MANAGEMENT application part or by the BSS TELECOM application part as the result of a system defense action which may be due to BSS equipment failures external to the BSC (e.g. RSL failure or CU recovery failure), or due to BSC internal hardware problems (e.g. TCU failure or DTC failure) or due to BSC internal software problems (e.g. inconsistencies detected between software modules or lack of software resources (memory, timer reference, file reference…) or communication problems between different processor boards).
most of the time, it is O&M actions, but it could also be boards of BSC of software of BSC failures. Check also your RSL congestion or quality ? (usually, if the RSL fails, the rest of the Abis also fails though)…
anyway, for this kind of sudden degradation, it is probably boards or software in BSC.