1998 Toyota 4Runner Starter? Alternator? Something Else?

'98 Toyota 4runner with about 130,000 miles on it. At unpredictable rates, my engine doesn't start or starts very funny. 10% of the time, it cranks very slowly faintly and then stops before it gets a chance to start. Another 10% of the time, all I get is a click and nothing else happens (I have to try it a few times until it does something at all). Another 10% of the time, it cranks slowly then "half starts" and the engine goes but at a very loud uncomfortable sound. The other 70% of the time it starts normally. My battery has been tested and is A-OK. Any ideas?