bad_ta_05

i have an 85 trans am and a 79 3/4 ton chevy truck 350 in it. my timing is advanced to 8* and i havent had a problem with it. it runs great adn everything but when u shut it off and then it sits for a little while it stutter starts and will barely turn over the 1st couple cranks. and now my oil pressure is going down again. i just changed teh oil sending unit and it fixed it and now its doing it again! i dont know whats going on. what could be my problem? what are the symptoms of a starter going bad? thanks alot