When I flush the bowl fills almost to the top, it swirls like its going to go down then stops about half way. Then, about 10 minutes later ALL of the water has slowly drained out. I'm finding answers to one or the other, but can't seem to find an answer to both happening. I tried to pour water from a bucket really quickly, but it just did the same thing.

You need to snake that drain out... starting from the toilet out. Youi have a partial clog. Possibly even something lodged in the toilet trap that was dropped into the bowl on accident.

I have one that most repair plumbers miss.. Look down at the bottom of the bowl. If there is a small hole, then that is a jet that starts the syphon action. If it's clogged the water will just swirl around and slowly go down leaving solids behind. Take your finger,(UGH! ) and run it around the inside of the opening. Over the years minerals build up and cut down on the syphon (flush) action. If it is rough or you feel build up, take a table knife and put a bend in it to get around the curve in the bowl and chip and scrap it clear. Next take a coathanger and clear out the holes around the rim. They start the swirling action. And last, check the water level in the tank. It should be 1/2" below the top of the over flow tube. And speaking of the overflow tube, Make sure the small 1/8" tube from the ballcock to the white overflow tube is connected so it discharges in it and that it's flowing when the ballcock fills. This is what raises the water level in the bowl. For a good solid flush they all have to work together. Hope this helps but if it doesn't then purchase a closet auger and snake the bowl. . Good luck, Tom