for sdcch drop, you can investigate the followings :
hardware/software problems (dynamic SDCCH ts allocation, TCU failures, RSL failures, TRX failures)
radio problems : Especially interferences on the frequency of the TRX’s that is carrying the SDCCH’s. Low coverage (low rxlev) can also have a big impact on the SDCCH drop rate (rural areas, mostly, as soon as avg rx lev of the cell is less than -88dBm)
Don’t forget to check the path balance of your “SDCCH” TRX’s.
handover problems : if SDCCH handover is enabled, then you might want to disable it and check the results.
You can also check the average duration of the SDCCH seizure : if it is above 5 seconds, then there might be “hanging” DTAP message during the SDCCH phase. If the SDCCH phase lasts too long, it increases the risk of drop.
That’s about it…
just a reminder : the SDCCH congestion has no effect on the SDCCH drop.