I suspect that the SDCCH drop is happening only in some cells ? not the whole alu network.
At what hour is the peak of sd drops ?
what is the SDCCH Mean Holding Time ?
can you get sdcch drop for location area updates only ?
The idea is to prove that those drops are due to a storm of location area updates from faulty mobiles.
On the opposite, if the drops happen on all cells, then you may be facing a wrong parameter configuration. Could you try to put all your SDCCH timeslots on the BCCH TRX ?
(ie. no sdcch ts on TRX other than TRX1)
Are you using frequency hopping “sfh” ?