Won't crank?

Ok, yesterday I took my car out around the block a couple of times and beat on it a bit. I left a few new stretches of rubber on my block, too. Fun. ;-)

Today I went out to start it up and it wouldn't crank. Just clunk. I had this happen last year, when I had coolant leaking into the oil. I thought it was leaking from the intake gaskets, so I assumed I had some water in the cylinders. After cranking the motor over by hand I was able to get it going again. I changed the intake gaskets and now the oil looks clean.

So today I had this problem, so I pulled the hood off and went to crank it by hand. It wouldn't move at first, then it went. No odd noises, just the usual compression hiss. Then I was able to start the motor normally (by using the starter, I mean). After I ran it for a little while I shut it down and checked the oil, it's still clean. Then it fired right up again.

Any ideas as to what could be going on? If there was something physical blocking the crank from spinning, it'd be making noises I would think... Could my starter just be weak? It's relatively new, only a couple of seasons old. The battery is strong too, I just charged it a week or so ago and it's disconnected when it's not in use.