I have the same problems as you, Cells with High SDCCH Fail Assignment Rate randomly appear for some hours duering the day.
Found T3103 timer expired when the problem is happening. I use GSM NORTEL 2G only.
Found some site alarms, and CSSR affected, SDCCH attempts almost doubled, seizures maintained as previous day without problems, TCH traffic assigned wo problems, no SDCCH congestion.
look at the forum for another post under “m@U” I have explained everything I have until now there.