I faced something similar a loooong time ago:
a BSC from competitor is down during the night, so all the BTS from BSC are down.
Therefore, all MS from competitor will attempt to get a SDCCH and perform an IMSI ATTACH. It generates a huge amount of SDCCH congestion.
Ok… it’s nothing like your current issue 🙂 but perhaps it can give you some ideas?
Anyway, can you tell me which timer expires? Could you write down the full message flow of what you’re seeing (on the abis for example)?
Bts – BSC : CHANNEL REQUIRED
BSC – Bts : CHANNEL ACTIVATION
Bts – BSC : CHANNEL ACTIVATION ACK
BSC – Bts : IMMEDIATE ASSIGNMENT
Where does it stop?