Hi,
reasons:
1- Faulty boards and Especialy the CDU,
2- Board slot on the BTS (Back plane),
3- TMU board,
4- BEU board or GEIUB in the new version,
5- Trans coder buses (between BSC and MSC),
those the main reasons and I already phase and its fixed after I changed GEIUB board one time and back plane in others,
good luck,
Regards