unresponsive betta

I've had my betta fish for over a year, she's always been healthy. Today she started seeming like she was problems breathing (gills moving abnormally), and she was in the bottom of the tank. Every once in a while she swims really quickly to the top, and then sinks all the way to the bottom...

Hi, I have had my Betta fish Benny for about 6 months. I adopted him from my neighbors who were moving. Since adopting him, I have taken better care of him than his previous owners. I change the water twice a week. Keep the temp between 74-80 degrees. Interchange his diet between pellets...