First, uninstall game
delete the SFC 3 folder off your system
reinstall SFC 3
Patch with Beta Patch from Activision's web site.. do not use the SFC 3 Direct X 9 Patch with Beta Patch as this will create game errors
check your video drivers, you may have to upgrade/ downgrade your drivers.. however if game is running correctly in single player mode, then that's not a problem...
what type of connection to the web do you have.. Dial-up, Cable, DSL? if Cable or DSL, what kind of modem do you have? If your DSL/Cable modem has both Ethernet/USB connection, disconnect the Ethernet connection and use USB connection.. this is due to those type of modems having built in routers on the ethernet connection, but not on the USB connection...
if you are on a home network, set your machine up as the DMZ machine and enable ports according to the Networking.rtf file included on the game CD (Browse CD and check different directories)
hope that this information helps you connect