most of the time, i find that SD drop is linked to interference from other cells or is caused by a large cell coverage (many subscribers at the cell border, with low RXLEV)
sdcch HO should be disabled, unless you want to make your life complicated. It will not improve the QoS in the end, so you’d rather leave it off.
if you want to make a better diagnostic : keep only on SDCCH timeslot, disable frequency hopping, and move your SDCCH timeslot on different TRX. Maybe only one frequency is interfered, or one TRX is faulty, causing trouble on the SDCCH timeslot.