Are you just adding mods or are you replacing the old files.Also how many mods have you installed(too many and it might crash). The only way I know how to fix it is to uninstall then install again. Thats what I did. Oh and one more thing do you have version 2.0 or 1.0. If 2.0 then you have to downgrade it. you can find out how to downgrade it by going to youtube and typing in HOW TO DOWNGRADE GTA SAN ANDREAS it should be the first one.