From what I understand the stock stereo is less then 10 RMS per channel. I would try and pick a speaker with an RMS between 0 and 50. Kenwood, Infinity and JBL have good selections.
My starter was doing the same thing a few months back. Then one day it wouldn't turn over at all. Had to hit the starter with a hammer to get it going again. Probably just time to throw a new starter in and be done with it.