DIY...Std shingled roof leak repair? Leak reoccurs dead center each rafter about 4 -5 times? What to do??

The wet spots are on the decking in the center of each rafter in the attic. The roof on the outside looks like new, no nails popping up or bulges or anything. I need a quik fix so I can finish the room to get the house on the market but I don't want to leave a problem either. If I push the nails in from the inside, find them on the outside and tar under the shingle where I find them shouldn't that stop the leak? I hope to stop the water, let it dry out and apply a sealer on the inside to protect the decking on the inside. Any thoughts, ideas or advise would be appreciated. I am having to do this myself, the pitch is steep and the roof line starts at least 14' - 15' up.