When it starts to die try pumping the primer bulb. If it acts like it wants to run, rebuild the fuel pump. Be sure to get the kit from Merc as it will include new style check valves.
you can also try adjusting idle speed and/or idle mix. you might be running rich and have a low idle, or either one individually. check mercruiserparts.com for your motor's S/N and it'll show you all the parts.