problem in antenna patterns would only generate SDCCH assignment failures or TCH assignment failures (well, sometimes the SDCCH assignment failures are counted into the SDCCH drops, by some vendors, check your formula)
sdcch drop between LAC’s occurs because of poor coverage at the border, so the Location Update is started but dropped due to poor radio conditions. try increasing your Cell Reselection Hysteresis and Temporary Offset (i.e. 12dB)and Penalty Time (i.e. 40s).
did you activate SDCCH handovers ? (disable them, by default)
you can also perform a drivetest/abis to see exactly what is the cause of the problem.