Hi,
I'm hoping someone can help me to figure out what is wrong with our toilet. Recently I flushed it and it overflowed. Then, when I tried to flush it again I noticed that the water just swirled and swirled, but never fully emptied from the bowl. I did a couple checks to help diagnose. If you have any advice, please share!

Here is a video of the toilet "flushing"
http://www.youtube.com/watch?v=rNfYgcQHcGI

What I've done so far...
1. Checked the water valve of cold water going to the toilet to refill tank and made sure it was fully opened so that water would fill in the tank quickly.
2. Checked to see that the water in the tank filled to the water fill line (it did).
3. Checked to make sure there was no leak from the tank to the bowl, that the flapper closed tightly and that there was no slack in the cord that pulls the flapper up (seem okay).
4. Poured 7 qts. Of water down the toilet quickly and saw that the toilet flushed completely. Here is the YouTube video of that process: http://www.youtube.com/watch?v=R9u-EB3n6wg

I read on this site that sometimes the siphon on the toilet doesn't work properly because of mineral buildup, etc. Do you think that is what is happening? If so, what should I do? And, if not... what might it be?

Thanks so much!